What is "Anapanasati" day which is coming on this November 14?

What is "Anapanasati" day? I used to fasting on Uposatha, I became unable to do after my health got down. Last time I had asked to what to do on Uposatha Day but as I mentioned above and still unable to do; but now I've to practice seriously and go to Vihara on Anapansati day so is it same as Uposatha ? And if it is have to practice, then can I go for it in the middle stage or from the beginning? But anyhow I want to practicing and feel spirit of it when I loosed of my life.