This is true, and it isn't just holidays. They usually start surge pricing on weekends after the T (subway for non-Bostonians) shuts down for the evening (around 12:30-1AM) and it continues until demand drops. That being said, if you try to request a pickup during a surge pricing period, there's a pop-up on the app that warns you about it and it gives you the option to cancel. It isn't something they tack on at the end, so surge pricing shouldn't come as a surprise to anybody.