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 a crime-fighting vigilante. She's a teacher by day and an actress at the local theater by night.
See also: by
Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.

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
McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002 by The McGraw-Hill Companies, Inc.
See also:
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.
Clinical and radiologic deterioration progressed, and by day 7, mechanical ventilation was begun.
In this study, "reduction in plasma HIV-1 of less than 0.72 log by day 6 after initiation of therapy predicted poor long-term responses in more than 99% of the patients." For those with less than a 0.96 log reduction, the chance of poor response at 12 weeks was 95%.
Physicians might want to look at viral load decline by day six of certain new treatments -- in order to quickly change a clearly ineffective regimen.
When the daily fecal Pb excretion was averaged across treatment groups, it was apparent that the majority ([is greater than] 96%) of unabsorbed fecal [sup.206]Pb tracer was eliminated over treatment days 1 and 2 and declined substantially to near 0 by day 5 in these juvenile monkeys (Figure 1A).
When stocks fall, one strategy used by day traders is the "Lizard Reversal." Traders watch for a stock that is going down all day, but at the very end it closes up.