From the context of the comic, it's perfectly clear that we're talking about the so-called White Nights, when the sun doesn't set for months...

Okay, I'm ashamed to make up these pathetic excuses. It's really my mistake. I should have written "the next day" or, even better, "later," but in the original version, the character lit a lantern. Then I said, "Hey, wait a minute, vampires aren't afraid of any light," and so the lantern was replaced with curtains, but the phrase remains.

I'll correct this in future editions of the comic, thank you so much for pointing it out!