How do you handle Thanksgiving & Christmas with 24-hour aides as far as time off, dinners, etc?

Contact the agency they come from and ask what they will be doing and whether they can provide alternative cover if necessary. Don't worry about what happens until you know there is something to worry about. Perhaps a period of respite in a memory care facility would be worth trying, I cannot think 24/7 home care is less expensive than a facility with staff all trained to look after ALZ patients. If she gets on well there then perhaps that is a way forward, if not then you have tried for a couple of weeks and ensured she is OK over Christmas and New Year.

pampie: I only had a short term pro bono caregiver until I could arrive from out of state to care for my late mother. Bridger46164 provides a good response.

Ask the aides what their plans for the holidays are. As a RN, my family and I are used to celebrating holidays whenever I am off. They may have similar arrangements with their families and significant others. If so, bringing a nice meal for your mom and the aide to share is a lovely idea. Check with the aides agency about gifts and such before buying them anything. If there is no rule against it, I suggest a gift card to local Walmart, Target or other store that tends to sell food and/or common department store items. Otherwise, we have sent fruit bouquets that everybody can share while on duty.

If an agency, confirm that they have people who work holidays.

If private pay, ask if they are willing to work on holiday - probably for more pay (when I used to work holidays, I got time and a half). You might offer to bring a fully cooked meal for the provider(s) who work on holiday and your mom. Just cook two dinners or cater the one at mom's house. And a couple of gifts would be nice, too.

Before getting aids on board - you might ask mom if she wants to attend at your house. Some elders still like the holidays. Of course if you have non-maskers or a really full house, it might not be safe for her.

My mother-in-law had caregivers through an agency. They came as usual for the holidays. I assume the agency gave them holiday pay.
My mother had private caregivers that she and I hired. I always took my turn once a week so I could have a good idea of what was going on with my mom. For holidays, I would work out a schedule that gave everyone some time for church and family. They did not ask for holiday pay, but they always got a Christmas bonus. The last couple of years, because of Covid and her mental state, I discouraged my sons and their families from coming for a family meal, so my husband and I would spend the day and eat the holiday meal with my mom and have the grandchildren over to our house for a family meal on an alternate day when I wasn't with my mom.
Last Christmas and Easter were just plain grim. My mom had no idea what day it was, and had eating issues that made mealtimes pretty horrible. The only reason I was there was because I felt it was my duty.

I have to pay my uncle's two aides holiday pay for Thanksgiving and Christmas. only the most senior employees get holidays off. He's in a nursing home and has an aide with him from 7am - 11pm. He is blind and likes to have someone with him to read things, go for walks, etc.

My aides do not celebrate any holidays so we are fortunate.
Husband and I will be eating a meal with mom and aunt and the sitters will be welcome to join us. Both seniors do not go out.

Have you hired them privately or are they through an agency?
If through an agency they may get "holiday pay" (most likely you will be charged extra for the holiday) It is also possible that you may not get your regular caregivers for the holiday if they are "senior" staff members they may get a choice of days off.
If you have hired them privately you can ask if they want the day off. If so you may have to find another caregiver for the day. If not I would offer bonus pay for the holiday. Probably Time and a half or double time.
What did you do on other holidays like Easter, Mothers Day, Fathers Day, July 4th (assuming you are in the USA) and did either of them ask for their birthday off, their spouses or a child's birthday off?

are they thru an agency?

I think it would be nice to give both holidays to enjoy with their families. But you can ask them. Maybe, and I doubt it, they get more money working holidays so would rather have the money.

Why can't you bring holiday dinner to Mom? Just a few people. You can always have the big meal Friday or the weekend.

Ask them!

I would think that a half day each or one gets Thanksgiving and one gets Christmas or maybe your family can step up and plan on spending both days with her so the aides can be with their families.