I'd say "early" means anything prior to June 1. "Late" means anything June 1 - Dec 31...1/2 and 1/2

OR it could mean 1/3, 1/3 and 1/3 (Jan-April, May-August, September-Dec)

I don't think one would want to break it into quarters...that is too specific for "early" "mid" "late"

"Mid" could encompass the middle third of the year (May - August)...