Calendar square (3)
(Other definitions for date that I've seen before include "6 [TREE]" , "Estimate age of" , "Romantic assignation" , "Rendezvous" , "eg 1588" .)
'calendar square' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for day that I've seen before include "Twenty-four hours" , "24 hours" , "24 hour period" , "Born yesterday" , "Dawn to dusk" .)