I used to hold it all in and never "talk back", but when I was at a family gathering and my macho-man type brother and dad were getting cranky cause my 3.5 year old fell and didn't stop crying when they thought he should, I told them all loudly and firmly to back off and leave him alone. I had NEVER done that before, and it felt great.
Speaking your mind is perfectly fine, though depending on family dynamics, some people can't handle it and don't want it to happen. The main thing is HOW you speak your mind. Some stuff you need to deal with yourself before bringing them out into the open. Other things are better left unsaid. If you have a therapist, many things can be said to them and dealt with first so when you do bring them up with the people that are involved, you've dealt with it yourself first.
Blowing up after holding something in isn't necessarily healthy because it's an anger reaction. Being firm or stern in a statement isn't blowing up. Try to deal with the stuff - not hold them in, but process it first, THEN deal with others who are part of the issue.
Don't know what to do re. Sunday - if everyone's going to be upset the whole time, it's not worth it. If you're going to feel under pressure, maybe you shouldn't go. But if you can be peaceful and try to deal with the tough stuff later, then go and make peace. Good luck!