Odds are good there won't be any newhire classes after mid-September, so that everybody is finished with training before Peak.
Peak is a busy time for EVERYBODY, new or otherwise.
Like any airline, if you're junior, you'll probably be working holidays. That being said, on Christmas Eve night aircraft operate into SDF and there generally aren't outbounds Christmas day, then early the morning of the 26th the system spins back up again. If you are based and live in SDF (or your family comes in), you can at least have ~24ish hours over Christmas Day with them.
Can't speak for how things work in ANC, ONT or MIA.