You could back out of the plan by saying you to need to celebrate a different day due to a schedule conflict. Then have your best friend also cancel. Reschedule a different day with just the two of you. Be aware that the other friend will probably find out and be very hurt. The other (nicer) option would be to just follow through, because you already agreed to those plans and it really isn't nice for that third person to get pushed out of something you already all agreed to. Just don't plan a shared party with this person in the future.

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