I've had serama hatch from day 19 to day 22 so be prepared for early arrivals. Most of mine hatch on day 19 and 20.
Yes, I'm prepared. My last ones hatched mostly on day 20. This is my first broody serama though, so I'll be more curious about when hers will hatch. I've seen big broody poop piles each day, and seen her off the nest briefly most mornings, but she has been very dedicated in the cold weather we have had.