Learn Swedish: Han rakar sig en gång om dagen.

In Swedish, frequency expressions (like "once a day" or "twice a week") use specific prepositions. You use om with words for day, 24-hour period, and year (om dagen, om dygnet, om året). For week, month, and hour, you use i (i veckan, i månaden, i timmen). It is best to memorize these pairs as chunks!

Why do we need the word sig here?

In English, you can just say "he shaves," but in Swedish, att raka sig (to shave oneself) is a reflexive verb. If you leave out sig, it sounds incomplete, as if you are waiting to hear what or who he is shaving. Since he is doing the action to himself, we must use the reflexive pronoun sig.

Why does the sentence use dagen (the day) instead of just dag (a day)?

When making frequency expressions with i or om, Swedish always uses the definite (the) form of the time word. So "a day" becomes om dagen (literally "about the day"), and "a week" becomes i veckan (literally "in the week").

Can I say en gång per dag instead?