"Vampires sleep in coffins during the day."
Translation:Vampyrer sover i kister om dagen.
under is used when something happens during an event:
Jeg kedede mig under middagen.
Han lukkede sin butik under krigen.
Hun skal drikke mange øl under festen.
om is used for specifying a regularly recurring time or period:
Times of the day: om morgenen, om eftermiddagen osv.
Seasons: om foråret, om vinteren osv.
Weekdays: om mandagen, om søndagen (on mondays, on sundays...)
I can't think any more uses for om in this sense.
om is here used to specify a recurring event ... but it does not necessarily have to be recurring. It could also just be to specify a time or period in which something occurs like "jeg ringede til ham om aftenen" (I called him in the evening).