OP - I think a lot of people are worrying about this type of thing, and I get your frustration completely.
I've been saving for a few things (aspirational items), and I completely caved yesterday and spent a very good chunk of that money on new land for Halloween, and so I could redesign to fit an item where I thought it would look like it was in the proper place my town (so bye-bye Sit'n'Rotate for the time being).
I agree with what most are saying - the stuff will still be there when Halloween is over, and you can still work toward it during the event (especially if they have event currency - that leaves your in game cash to build up a bit). And, you will probably find that there is time to do both the event tasks and the regular tasks. Think of it as something to work toward, and not as being left behind (I know, easier said than done).