Nikon D780 Reference Manual page 430

A
Calendar Playback
During calendar playback, you can delete all photographs taken on a
selected date by highlighting the date in the date list and pressing the O
( Q ) button.
A
Deleting Copies
If the picture selected in the playback display
when the O ( Q ) button is pressed was
recorded with two memory cards inserted and
[Backup] or [RAW Slot 1 - JPEG Slot 2]
selected for [Role played by card in Slot 2],
you will be prompted to choose whether to
delete both copies or only the copy on the card
in the current slot ( 0 281).
384 Playback