When asked how many seasons we have or how long each season lasts, many would argue 4 and that they last 90 days each. In Tibetan and Chinese medicine, we say that there are 5 and that they each last 72 days. Where the difference comes from is aligning the 5 seasons with the 5 elements/phases of nature: Wood, Fire, Earth, Metal Water.
Wood is associated with spring; Fire with summer, Metal with autumn and Water with winter. Earth is seen as mediating and bridging each season together harmoniously. Thus, the 18 day period between summer and fall, between fall and winter, between winter and spring and between spring and summer is earth.
So, typically in the lunar calendar of Tibetan and Chinese medicine, when we are starting winter according the western calendar on December 21st, winter is actually half over. The shortest day of the year (the least light), marks mid-winter. As our bodies are a microcosm of our world, this becomes very important and instructive on the kinds of behaviors and lifestyle that we should be following. And, as clinicians, it is equally important that we advise our patients appropriately and that we do not misinterpret signs on the pulse and symptoms incorrectly.