Upvote:2
You will NOT go through customs in San Francisco.
All Canada->US flights go through US immigration/customs in Canada before you depart. On arrival in the US you are basically treated as a domestic passenger - no need to go through customs/immigration, no need to re-clear security (presuming your flights are not in different terminals that are not connected airside), and no need to collect your bags presuming they have been checked all the way through.
In your specific case flying United on both flights you will land in SFO in either Terminal 3 or International Terminal G, and your outbound flight will also leave from one of these two terminals. These terminals are connected airside, so at worst you will need to walk between them which is at most about a 15 minute walk. United will check your bags all the way through, so you will not need to worry about them in SFO.
1.5 hours is plenty for this connection, presuming that your inbound flight isn't more than an hour or so late.
December 25th is normally one of the quieter travel days of the year, although that might be a little different this year given it's on a Thursday.