by day

by day

During the day. Often used to refer to one's employment or other typical daytime activities, in contrast with what they are apt to do "by night." By day, he was an average office clerk, but by night he was crime-fighting vigilante. She's a teacher by day and an actress at the local theater by night.
See also: by

by day

 and by night
during the day; during the night. (Often used in simultaneous clauses for contrast.) By day, Mary worked in an office; by night, she took classes. Dave slept by day and worked by night.
See also: by
References in periodicals archive ?
Based on the needs assessment conducted by ACA in the fall of 2003, several critical needs were expressed by day camp directors.
When stocks fall, one strategy used by day traders is the "Lizard Reversal.