So, basically, December 24, Christmas Eve, will have the buses and trains running on regular weekday schedules, so don't worry about Friday.
Saturday, December 25, Christmas Day however, Buses and Max will be on Sunday schedules, so if your bus doesn't regularly run on Sundays, tough luck! It won't be running at all this weekend. (Don't worry, you're not alone. My two regular buses, lines 51 and 67, both don't have Sunday schedules). The black hole money waster called Wes will not be in service as it does not have a Sunday schedule and doesn't normally run on Sundays.
The Portland Streetcar is using a reduced Sunday Schedule for Saturday, December 25th that can be found here:
(ha, these are reduced?? They run every stinkin 17 minutes!...ridiculous....)
Alright and finally, to top it off, TriMet's Customer Service office and Ticket Office at Pioneer Square will be closed on Christmas Day.