Would it be worth doing in calls at x mass?
You asked about incalls.
Lots of people advertise both but that doesn't mean they will necessarily be busy.
My advice would be to focus on areas where the local demographic doesn't celebrate Christmas. I'd need to Google this myself as I have no idea what religions do/don't.
My only experience is between xmas and New Year. One of my friends was very busy at this time, whereas I wasn't.
The other thing to bear in mind is the limited services in hotels over the festive season. The (chain) hotel I was in only offered a limited menu and it was crap. Fuck that. I need my grub.
If it's Outcalls from your home location, the only thing to do is make yourself available and see what happens. Assuming you already offer Outcalls, you'll have the necessary safety checks in place. The freaks are out all year round!