Day 18, as Aart said. Since you set on the first of the month, day 18 would technically fall on December 19. However, it shouldn't harm anything to lock them down a day early, or a day late for that matter, so don't stress about being exact!
Don't worry one bit about not being on! We all have stuff in our lives that's more important than chatting. I'm sure everyone understands! 😊
Glad to hear that the eggs are developing well, though! Go, eggies, go!! :wee
I'm usually wrong on who will hatch first from my own birds' eggs, let alone someone else's, so I'll just watch (and quietly cheer on egg K since that's my first initial, even though that egg is porous 🥺 ).