Do any of you know a church in the Los Angeles area that would do weddings on a Sunday?
I'm in Hong Kong and my fiancé is from LA. I've just found out that very few (if any) Catholic churches do weddings on a Sunday. We have already booked the reception venue for December 8th (Sunday) and all of their Saturdays close to that date are fully booked

I wish I knew that about the churches in LA earlier, I would've picked a different date!
Re: HELP! Catholic Wedding on a SUNDAY?
My wedding is a Sunday (Oct. 13), and we had difficulties. The priest of the church where we are getting married (in my hometown) was more than happy to let us use the church itself, but he straight-up told us he couldn't marry us because he had a wedding the day before as well as weekend Masses. The priest who's doing our pre-Cana is able to marry us, but he has to travel 2.5 hours the day of the wedding to do it.
You don't say this in your post, but which of you is Catholic, you or your FI? If it's your FI, he should be talking to his parish priest about getting married there. Also, you both need to look into pre-Cana, because it's a process to get your ducks in a row for a Catholic wedding. Most parishes recommend starting the pre-Cana process (getting sacramental records, getting records from other parishes, etc.) nine months in advance. If one of you isn't Catholic, you'll need to figure out that aspect, too.
I agree with PP. Either unbook this venue and try again, starting with finding the church first, or get married on Dec. 8 and resign yourself to not having a Catholic wedding.
Also, I just realized -- Dec. 8 is a holy day of obligation; you can't get married on that day regardless. The church has strict prohibitions on Sunday weddings. Your date is off-limits because it's both during Advent AND a HDO. Sorry to break that to you.
December 8th is the feast of the immaculate conception, a holy day of obligation. you will never find a church willing to do a wedding on this date. id change it ASAP.
You've gotten yourself into a pickle here. But the only thing you can do is move forward and do what you can. I have two options for you to consider.
1.) Can you change the reception to the day before? Are you able to switch with the date with your venue and not lose your deposit?
2.) Keep you wedding the day you've planned, and not have a Catholic wedding (as disappointing as this would be to consider). I would do everything you can to avoid this, especially if having a Catholic wedding is important to you.
As a guest, I would not like having the ceremony and reception on separate days, so I suggest that you avoid this as well.
I wish I had better advice for you, OP. Good luck!