While companies may call on any day of the week with a job offer, Tuesday is statistically the most frequent day to send out offers, followed by Thursday. This is most likely due to the same reason why recruiting managers seldom call at 9 a.m. with a job offer. They need to spend the early part of the day catching up on work over the weekend, so Monday will be a busy day for administrative activities. Tuesday may be less hectic, allowing the recruiting manager more opportunity to contact out.
Another common day is Thursday, most likely because the recruiting manager wants to reach out before the weekend but doesn't want to wait until the following week. Instead of possibly wasting office hours, Thursday job offer calls allow the applicant plenty of time to ponder the offer over the weekend.