On the idea of a seasonal regimen
The classical calendar divides the year into six ritus of roughly two months each — shishira, vasanta, grishma, varsha, sharad and hemanta. Ritucharya is the name for the regimen described against each of them: what is eaten, when one rises, how one bathes, what is worn.
The scheme is a description of a particular year in a particular place. It was written where six seasons could be told apart, and it reads as an account of that year rather than a rule for every year.