Board Thread:General Discussion/@comment-3146749-20160831222311/@comment-1859034-20160901011704

Probably sometime around "Merry Christmas Mordecai", though it could've been as early as "Real Date". They've had a mutual respect for one-another since "Camping Can Be Cool" (with Rigby revealing his true feelings in "Diary"), so it's taken a little while for them to move to the next level.