The list of Imps has now been updated with which time they are to be found in the chests:
Autumnal Imp L - DEEP (August/September)
Imp Of Love L - RAGE (February/March)
Spooky Imp L - DEATH (October/November)
Spring Imp L - STEALTH (April/May)
Summer Imp L - SUMMER (June/July)
Winter Imp L - STOIC (December/January)
Source: