Saturday 2nd May
The unsealed queen cells I left in the hives will be closed up by the workers on the eggs' ninth day since being laid. Inside this sealed cell the larva pupates like a caterpillar in a cocoon and the adult queen hatches out on the sixteenth day.
This is a much shorter development time than any other bee in the colony because of her luxury diet - she is fed nothing except for pure royal jelly for the whole first nine days, whereas workers and drones only receive this for the first three days.
Judging by the size of the larvae I left in the hives on Sunday, I think they were probably about six days old. If this is the case then the cells will have been sealed shut on Wednesday and the queens should hatch out next Wednesday.
I'll allow a few extra days in case I estimated the larvae's age incorrectly but if we can check them on Sunday 10th May we will be hoping to find two newly-hatched queen bees...