Who usually gets them the weekend of the 21st?? We don't do any special "holiday break" schedule and usually treat the weekends as part of the regular schedule unless the holiday actually lands on a weekend. We do the same with our weekly overnight schedule. My XH has never requested to have the kids extra just because it's a school/summer holiday. He's offered to pick them up a little early if he gets off work early or send them home later if he has a day off, but that's it.
This year, the holiday is right after a regular Daddy weekend, so my kids will be with him from the 22nd (his weekend starts at 8am on Saturdays because he wants his Friday nights free) to the morning of the 25th - he always has them all day on Christmas Eve until about 8am on Christmas Morning (because that's how it was spent when we were together - his family on the eve, mine on the day). Plus, his dad's family always does a party the weekend before Christmas, which happens to be late this year so it all works out.
my kids are 8 & 12 and this is the 5th holiday since we split up. When they were littler (up to 2 yrs ago when XH started dating and wanted more non-parenting time), we didn't go more than 2 days with them being away from either parent.
Sorry I can't help more, but maybe this gives you some ideas.