[quote=@Hael]
<Snipped quote by Rtron>

Do the monsters only return on that one week of the year, or every night the sun sets year-round?
[/quote]

Monsters return every night year round. When the sun rises, they have to flee. 

For an entire week once a year, the sun doesn't reach Galbar, due to Vestec's eternal darkness. The monsters have free reign for 168 straight hours.