You're in VA, which means you're covered by the Atlantic Area supplement. You have to work 30 days within 90 consecutive calendar days to attain seniority -- days work in November/December do not count towards seniority. If you were hired before Nov. 1, the days before November do count towards making book.
As far as joining the union, you should approach your steward and/or call the union hall. Dues won't be deducted until you make seniority. As a seasonal, you "fell between the cracks." You're in a right to work state and regular hires are given a 15 minute orientation by a steward with the opportunity to join the union; this usually doesn't happen during orientation for seasonals.
If you show up to work after you make seniority, you're guaranteed 3.5 hours of work. If the company is trying to give you a one day lay-off, they must inform you by telephone call prior to your start time. If they fail to do so, grieve them for your guarantee every time. Additionally, if it fits in your schedule and you work in a hub, you might want to consider transferring to the local sort ASAP (Day, Twilight, Midnight) as there are fewer problems with being laid off on the sorts.