What is a proper amount to tip a massage therapist for the holidays?

You should tip at least 15 - 20 percent. Stiffing your therapist on a holiday is the worst kind of rude. If you show up late and don't get your full time on a busy day like valentines day, that is your own fault. If you get a gift certificate and the tip is not included, that is also no reason to not tip. Massage therapists like waiters depend on tips as part of their payment.