No matter how far in advanced you talk to all your “attendees” there will always be a few people who decide that your event/get together isn’t as important as the new plans they just made with someone else based off of the same concept. Such as two people having a Halloween party on the same night. Odd’s are, only your closest friends will show to yours, even if you ask everyone else in advance.
I find it irritating because you went through all the time to plan it that far ahead and someone else walks in the week before you event and goes “HEY! i’m having the same type of event as them and on the same night, come to mine instead of theirs!” It’s like, why did i even bother?