coffins - more units during daytime!
 

tl;dr: looks like if you open vampire coffins during daytime, you'll get more units out of them. sorry if it's not new information for anyone but I haven't found any info on this, so I thought I'd share.

here's what happened. 1st visit to cemetery, I was just running around, exploring, grabbed a few coffins, got some vampires, skeletons, whatever.

reloaded, started doing the area seriously (no fighting yet, just grabbing what I could), managed to get 3 coffins. opened them, got 3 vampires, decided it's not a lot, I'll use them later, so I reloaded again.

a bit later, back at the greenwort castle I checked the coffins again and that time I got 12 vampires from the same 3 coffins. now that's better but I still didn't need them, just wanted to check the stats, so I reloaded again.

a tried opening the coffins a few more times to see if it's random or what and I got 6 vampires in the forest but when I got back to greenwort, I only got 3. I was pretty pissed, I mean 12 vamps are better than 3 or 6, so I started googling how it works but didn't find anything.

then I had an idea to check an earlier save where I got 12, and when it worked I realized that it was daytime then, when I got 6 it was getting dark and I got 3 at night. pretty neat.

I also tried it with regular coffins, got 12 skeleton archers from 3 coffins at night and 15 during daytime.