We had one foal born during a storm, a big thunderstorm. Spunky black as night filly with a star, named her Stormy and she was a handful from day one. Of course she went to a light grey by the time she turned 6 years old
The worst time is late in the year. You want to have it as close to JAN as possible without going into DEC.
A horse turns a year older on JAN 1st. So if the horse is born in DEC it will be a year old even if it is 1 day old.
I try to breed for a April foal.