It is about shifting bidding, not getting paid more. Some senior pilots would intentionally bid the holiday trips for the premium pay. It isn't just for the "junior" pilots either. I've seen some categories where coverage awards over the holidays reach the 50% line.

Although not pilots, this example proves the point. I recently flew two international 3 day trips connecting the exact same city pair. One trip left on July 4th (Thur-Sat), the other on the 7th (Sun-Tues). Although both trips touched the weekend, the seniority of the FAs was MUCH higher over the holiday trip. I even heard a few comments about how they specifically bid for the trip due to holiday pay. Three of the junior FAs on the first trip, returned as the senior FAs on the following trip.