This is the hinge the classical texts warn about most loudly, and it is worth knowing why.
Three things happen at once. The Vata that quietly accumulated through summer's dryness now meets cold, damp and wind, and aggravates — this is prakopa, the breaking-out stage. Pitta begins its own silent accumulation, banked for autumn. And digestive fire, already weakened by the whole Adana half of the year, reaches its lowest point. Charaka is blunt about the cause: the earth's vapours, the sourness of the water, and the cloud-covered days between them leave agni with almost nothing behind it.
So the monsoon is the one season where people reliably eat as though nothing has changed and then wonder why they feel heavy, gassy, achy and low. Nothing has changed except the one thing that matters — how much you can actually digest.
The sandhi rule applies here as it did in March. Give the old regimen a week to taper and the new one a week to arrive. Do not go from summer's cold sweet drinks to monsoon's warm sour-and-salty in a single day; the texts call an abrupt switch a cause of disease in itself, not merely an inelegance.
What to actually do in these fourteen days: bring the temperature of your food up. Cooked, warm, light, a little oily, a little sour and salty. Add ginger before meals rather than after. Drop raw food almost entirely — the monsoon is the classical argument against salad, and it is a digestive argument, not a hygiene one. Boil your drinking water and drink it warm. Keep your feet dry and warm, because damp feet and Vata aggravation are the same conversation.
And start eating a little less than you want to. That instruction sounds austere until you feel how well it works this month.