Oh Dear...

Given that brummytom's the only person who thanks my worthless posts, I'll impart this advice from my considerable years of experience:

When things like this happen, there's always a right and a wrong way to do things. The wrong way inevitably results in everyone blaming you for ruining what should have been a great family occasion. The right way results in more presents.

What I would do is:
1. forget a problem ever existed and get distracted by something on boards/other website;

2. as the time for the occasion draws closer, get a niggling feeling that there's something you should be doing;

3. Somewhere between a week and two days beforehand, my mother/father remind me what it is I should be doing;

4. Forget there's something I should be doing;

5. Get drunk... remember... go to both events and try and score your mother's best friend (again).

Note: THIS IS THE WRONG WAY OF DOING THINGS.

What you should do:

1. Don't mention this to your mother, because if she has to fake it, you're not going to be able to think of anything except, "that's how I'm here now". Don't tell your father to go along with her plans because he cannot fake things. Again, this will poison your thoughts for the night;

2. Suggest to your mother that she keeps the night in question for the two of them and that she postpone her party idea for a week. She'll agree that it's far more romantic to let their anniversary be just the two of them. (Bonus points for Tom.);

3. Ring all the mutual friends of theirs you can find and tell them what's going on. That way, when she invites them, they can accept in the full knowledge that they're really going to be surprising her a week earlier, negating the need to go to the next weekend's celebrations;

4. When steps 2 and 3 are completed, tell your father about it. This will make him feel manly for two reason: (i) he's got one up on his wife and (ii) he's raised a clever son. (Bonus points for Tom.);

5. Watch as your plan slots into place - your dad gets to surprise your mother and your mother thinks he's the bomb for organising everything. He'll take the credit for a few days until, guilt-racked, he confesses that it was all your doing. You have now impressed both parents;

6. You get more/bigger presents.