In Ayurveda, Ritu Sandhi means the "joining" of seasons. You say it like this: RIH-doo-SAHN-dee.
When the seasons change, there's a period of transition. Think of it like a gap or a joint between the seasons.
When the seasons change, we are susceptible to sickness because the doshas that rule the season are also changing. Therefore we have to adjust our eating and lifestyle routines from one season to the other as a transition. Our bodies have been used to the routine, food, and lifestyle of the last season and without some prep work, switching to a new season immediately is a shock to the system which affects our immunity.
In Ayurveda, the three doshas govern specific seasons.
Pitta governs: Summer
Vata governs: Fall thru Early Winter
Kapha governs: Late Winter thru Spring
Knowledge of what dosha rules which season is super valuable information because we can (and must) adjust our diet and lifestyles accordingly to avoid getting sick. For example, in summer we crave light and cooling foods, which balance pitta's heating nature. In the winter, we need heavier, oilier comfort foods which balance vata's cold and light qualities. In the spring, when it's damp and rainy, we need dry, light and spiciness in our foods.
Sounds pretty simple and intuitive, doesn't it? Great. That's what Ayurvedic principals are all about...using opposites to balance the doshas.
Below is a chart that shows how the energies increase and decrease during a year's time.
Notice that when the curve dips, the previous dosha is low while the next dosha is just ramping up? This is ritusandhi and when you want to start getting ready for the upcoming season. How do you do that? I'll tell you.
1) Consider doing a kitchari cleanse for 3-5 solid days to lightly detoxify, reset and rebuild the body-mind with renewed cells and tissues. And yes, I do recommend doing this 3x a year! It's the cheapest and most effective way to give our body a well needed "oil change" at home.
2) Incorporate foods that are appropriate for the upcoming season while weaning off foods from the last season. Nature is pretty good at setting us up for this. You can shop farmer's markets which are almost always harmonious with nature and seasons.
3) Change your exercise routine to match the season, if necessary. Cooling exercises like biking, swimming outdoors, and surfing should be done in warmer months. Whereas in the winter, cross country skiing keeps us toasty warm/sweaty on the inside and keeps both feet on the ground. Yoga is good for all seasons :)
4) Keep in mind your dosha! If you are vata, you are almost never going to be drinking ice water! If you are a pitta, you'll always want to avoid anything too spicy or fermented. If you are kapha, you will always want to avoid too many sweets.
5) Stick to the daily routine...it's nature's law! The only thing that shifts is sunrise and sunset. The rest is to be followed as a daily habit.
The Seasonal Blooms will give you awesome information and details about each dosha and each season. Get a copy, they will help you!