It's not strictly because of the day off. That's just one of the variables in the equation. But it makes it a bit easier since some of the concern with not rotating is fatigue. If Swayman starts, it'll be 2 games in 4 days. That's not unusual, they have 3 games in 4 nights during the regular season where one of them has to play 2. Also, it's a small sample but when Swayman played consecutive games this year he posted a .923 save percentage in the second game.If you ditch the rotation because of the extra day off, you ditch it for the duration because you are making a definitive statement about your confidence.
And Ullmark has done nothing to shake confidence in his play. They lost game two because of stuff outside the blue paint, not in it.
I'm also firmly in the camp that sees these goalies as better individually because they are not alone carrying the burden.
If the Bruins go into game six with a 3-2 lead and two Ullmark losses, I'll reconsider. Until then, I'd just stick with the rotation.
I don't have a strong preference and agree with @kenneycb that it's probably 51/49 but I would lean Swayman. If it doesn't go well or they see a dip, there's nothing stopping them from using Ullmark in game 5.