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: