The sixth chapter of Sutrasthana of Charak Sanhita explains the seasonal regimen in detail. It is called as Tasyashiteeya Adhyaya. It literally means – qualitative dietetics explained based on seasons.
Shloka recitation video
Importance of the knowledge of Dietetics
The strength and lustre of the one who knows about suitable diet and regimen for every season and practices accordingly get improved and enhanced. 
The two solstices in an year
An year comprises of six different seasons. 3 seasons each are grouped into two solstices.
Hot and dry. The northward movement of Sun brings about water loss in the body beginning from late winter to summer. Sun is dominant.
Cold and wet. The southward movements of Sun is coolant and forms the other seasons beginning with the rainy to early winter. Moon is dominant.
Hence one has to adjust his diet based on these variations. [4-5]
Effect of Adana and Visarga Kala on body
Effect of Adana Kala on body
Uttarayana – Adana kala – Northern Solstice – mid January – mid July
During this period, the strong sun rays and pacy winds bring about dryness and absorb moisture from the earth. The three seasons of this period are –
Shishira Rutu – Magha and Phalguna (Mid January – Mid March) – late winter. This season enhances bitter taste and body strength.
Vasanta Rutu – Chaitra and Vaishakha (Mid March – Mid May) – spring – enhances astringent taste – lower body strength
Greeshma – Jyeshta and Ashadha (Mid May to Mid July) – summer – enhances pungent taste, lowers body strength.
Due to increased dryness, the body becomes weak. 
Effect of Visarga Kala on body
Dakshinayana – Visarga Kala – Southern Solstice – mid July – mid January
It has three seasons.
Varsha Ritu (Rainy Season) – Shravana and Bhadrapada – Mid July – mid September – enhances sour taste – mild body strength.
Sharath Ritu (Autumn season) – Ashvayuja and Karthika – Mid September to Mid November – enhances salt taste – moderate body strength
Hemantha Ritu (Winter season) – Margashira and Pushya – Mid November to Mid January – enhances sweet taste. – high body strength.
The oiliness / unctuousness in the body grows in these periods and dryness is relieved. Hence body strength increases in this period. [7-8]
Winter season (Hemanta Rutu)
During winter, the digestion strength is so much improved that it is capable of digesting any food stuff irrespective of its heaviness and the quantity.
During winter, if proper quantity of food is not taken, it affects the quality of Rasa Dhatu (nutritious fluid generated as a product of digestion). It leads to Vata imbalance.
Therefore, during the winter one should take foods having the following qualities –
Snigdha – oily – unctuous,
Amla – sour and
Lavana – Salt.
Audaka mamsa – meat of aquatic animals
Anupa mamsa – meat of animals from marshy place
Medya – meat with fat.
Bileshaya Mamsa – meat of animals dwelling in burrows and
Prasaha – animals which catch food by teeth, tear and eat
Madira – wine
Seedhu – fermented liquid prepared with ripened fruit should be consumed with honey.
Gorasa Vikruti – cow milk and its products
Ikshu Vikruti – sugarcane and its juice products like sugar
Vasa – fat of animals
Taila – sesame oil
Nava Odana – fresh rice and grains
Ushna toya – hot water
One should follow the below regimen during winter –
Abhyanga – oil massage
Utsadana – mild massage using the palms
Murdhni taila – oil application to head
Jentaka Sweda – a type of sweating therapy where one resides in an underground home
Atapa – exposure to sunlight.
One should ensure that conveyance, bedding and seats are well covered with heavy wrappers, skin, silk cloth, ropes and blankets. One should wear heavy and warm clothes.
Should smear his body with Agaru – Aquilaria agallocha.
One should embrace a healthy woman with big breasts, who has anointed with Agaru (Aquilaria agallocha Linn); then he should lie down on the bed intoxicated with strong passion.
One may indulge in excessive sexual intercourse during the winter. One should avoid foods and drinks which are light and are prone to Vata vitiation. One should not expose himself to cold wave.
Eating less and intake of Yavagu (gruel) should be avoided. [9-18]
Late winter regimen – (Shishira Rutu)
The Hemanta (winter) and Shishira (late winter) seasons are similar. But the dryness of Adana kala (shishira falls in Adana Kala) and cold caused by clouds, wind and rain intensifies. So the entire prescription for Hemanta (winter) is to be followed during Sisira as well. One should stay in a windless and warm home. One should avoid taking diets and drinks as are possessing pungent, bitter and astringent tastes which cause Vata imbalance. One should avoid food that are light to digest, food having coolant property and cold drinks. [19-21]
Spring regimen (Vasanta Rutu)
During spring, Kapha that is accumulated during the winter gets liquefied by the heat of the sun and disturbs the digestion power. It leads to many diseases.
So, one should undergo therapies like Vamana (vomiting therapy)
One should avoid
Guru – heavy to digest foods
Snigdha – oily, unctuous foods
Amla and Madhura – sour and sweet foods.
One should not sleep during day time.
At the advent of spring one should habitually resort to
Vyayama – exercise,
Mardana – Palm massage / unction,
Dhumapana – smoking,
Gandusha – gargling and
Anjana – collyrium.
The excretory orifices should be regularly washed with lukewarm water.
One should smear his boy with Chandana (Sandalwood) and Aguru ( Aquilaria agallocha).
One should take the following foods –
Yava – barley
Godhuma – wheat
Meat of Sarabha (Wapiti), Shasha (Rabbit), Ena (Antelope), Lava (common quail) and Kapinjala and Mridvika (raisins) types of wine. One can have sex. [22-26]
Summer regimen – Greeshma Rutu
During the summer, the sun rays evaporate moisture of the earth. One should have
Madhura – sweet tasting foods
Sheeta – coolant foods
Drava – liquid foods
Snigdha – oily, unctuous foods and drinks.
Cold Mantha (mashed fruits with sugar)
Jangala Mriga Pakshi – meat of animals or birds of windy climate,
ghee and milk along with rice.
Avoid alcohol during summer –
One should either drink alcohol in small quantities or should not drink at all. Even if someone consumes, it should be diluted with plenty of water.
One should avoid taking
Lavana, Amla, Katu and Ushna – food having salt, sour, pungent and hot qualities.
Vyayama – Physical exercise should be avoided.
During day time one should sleep in an air-cooled apartment. During night, after smearing body with sandalwood paste, one should sleep in open airy roof, which is cooled by moon rays.
One should wear pearls
One should avoid sex and should enjoy gardens, cold water and flowers during summer. [27-32]
Rainy season regimen (Varsha Rutu)
During rainy season, the body is weakened due to weak digestion strength. It is further weakened due to Vata imbalance, which vitiates body tissues.
The water gains sour taste due to clouds, cold wind and mist.
One should avoid
Udamantha (beverage prepared with flour of corns mixed with ghee)
Divasvapna – day sleep,
Nadi Jala – water from rivers,
Vyayama – exercise,
Atapa – exposure to sunlight and
Vyavaya – sex.
One should generally use honey in preparing diets, drinks and others.
If the days are cooler due to heavy rains with storms, one should take diets that are conspicuously sour, salty and unctuous; this serves as an effective antidote to the vitiation of Vata during this season.
In order to maintain normal digestion strength, one should take
Purana Yava – old barley,
Godhuma – wheat and
Shali – rice
Jangala Mamsa – meat of animals from arid region. It should be processed with Yusha (vegetables soup).
Madhvika or Aristhta (alcoholic preparation) along with honey and water should be consumed.
Pure rain water or water from the well or pond, boiled and cooled, mixed with a small quantity of honey should be consumed.
Pragharshana – massage body with palm
Udvartana – powder massage
Snana – bathing
Gandhamala – wear fragrant garlands during the season.
One should wear light and clean clothes and should reside in a house devoid of humidity. [33-40]
Autumn season regimen – Sharath Rutu
The body parts adopted for rains and cold are suddenly exposed to the heat of the sun with the beginning of autumn.
The Pitta accumulated during the rainy season gets imbalanced.
In this season, sweet, light, cold, and bitter foods and drinks should be taken so as to tackle Pitta.
One should have
Lava – meat of common quail,
Kapinjala (grey partridge),
Sharabha (Wapiti), and
rice, barley and wheat.
Tiktasha Sarpisha Pana – Intake of ghee processed with bitter herbs,
Virechana – Purgation therapy,
Raktamokshana – blood letting are advised.
One should avoid
Atapa – sun bath,
Fat, oil and meat of aquatic and marshy animals and
Kshara – alkaline and lavana (salt) preparation and
Dadhi – curds.
One should not sleep during day time and one should not expose himself to frost and east wind.
The water that is exposed to the heat during day and moonlight during night and purified and detoxified by Agastya star is known as Hamsodaka. It is very clear and is good for bathing, swimming and drinking.
Use of garlands made of autumnal flowers and clean cloth and also the rays of the moon in the evenings are very beneficial. [41-48]
Thus the seasonal regimen and diet has been explained.
When a person indulges in wrong food / lifestyle habits for a long period of time, that wrong habit becomes habitual to the person. This is called as Oka Satmya. Though it is wrong, it might not cause much harm to the person, like the poison in a snake. 
One should consciously resort to food and lifestyle habits that are opposite to the nature of the season and of the disease. 
Thus ends the sixth chapter on Tasyashiteeya chapter of Sutrasthana of Agnivesha, as redacted by Charaka.