"Hej" translates to English as "hi" or "hello". It is a common polite greeting that would be used in most situations. The exception would be very formal occasions, but Danes pride themselves on being friendly and informal, so "hej" is fine to use in most situations. "Dav" or "davs" is less commonly used and is an abbreviated/informal version of Danish "goddag". "Goddag" literally translated means "good day" and is used from late morning when "good morning" would be used in English through the day when "good afternoon" would be used. "Dav/Davs" would translate to English as "G'day" or "Howdy".

I see that LoneC said what I would say about dav being a bit like "howdy". Countrified and old fashioned sounding. Hej is a cool word. Say it twice and it means goodbye.