As far as I have been lead to believe Medicine Buddha day is every "last quarter".
New moon day is usually Shakyamuni Buddha day, but hell, if you celebrate Sangye Menla yah can't go wrong! Essentially, all Buddhas, have one essence.
"When one is not in accord with the true view Meditation and conduct become delusion, One will not attain the real result One will be like a blind man who has no eyes." Naropa - Summary of the View from The Eight Doha Treasures