If months were numbers (1 to 12), you could add +12 to any month below the month you are setting in priority. For example, let's say that Jan=1, Feb=2, ... Dec=12.
If you specify "sort from March", then Jan=Jan+12, Feb=Feb+12, Mar=3, ... Dec=12. So after sorting, Mar=3,...,Dec=12,Jan=13,Feb=14.
From there, you should figure out what to do with your comparator.