Hi Egbert, I'm not too sure when the best time is, when we were just starting with chooks our neighbour said over the years he'd bought his at different times of the year and never had a problem.
We thought it would be best in the spring but when we had finished the run at the beginning of February 3 years ago I couldnt wait and we went straight out and bought our first 6 ! They were fine, even though it was really cold, and although 1 died (large egg which she couldnt lay, we tried to help but lost her) the rest are still doing well.