I agree w/ Cali. You're just making it a power struggle rather than a cooperative partnership. Let her have the day for the church event. Wish them a great time. There will be times where you want the extra time for some specific similar activity. If you've established a pattern of flexibility and reasonableness, and not needing everything to be you get one, so I need to get one, you'll probably end up ahead.
Instead of worrying about the extra day & fuming over her response, just look for something extra fun to do when you get the kids back. My 2 cents.
Me: 50 W:43 S6, S3 M: 12 yrs. T: 17 M is bad & Not happy Bomb Mar '14 S 5 Feb '15 D Bomb 13 Apr '15 (but "no hurry") DB Coach May '15 Wants proceed on D Aug '15 Starting 1-on-1 negotiations Sept '15