It is a goodwill gesture, a gift. The company doesn't have to do this for anyone, it isn't discriminatory, it isnt an established custom to give everyone a day off. It is just dumb luck.
If Christmas Eve was Thursday they probably wouldnt give anyone the day off, they've probably just done this because it makes a lovely long Christmas weekend...which you are presumed to get anyway, as you don't work Friday!
If you happened to work Fridays, you'd get the day off, so that's not discriminatory.
If an FT employee is off sick on that Christmas Eve Friday, they wouldn't have a right to ask for another day in lieu because they couldnt enjoy the gift.
If the company decided to buy everyone pizza next Christmas Eve on your day off, you wouldn't go in and complain you missed the pizza, and please could you have a pro-rata 80% share of a pizza on one of your working days, and enjoy a lunch break extended by 80% of the extra time thr FT employees were given to eat the pizza? It would be absurd.
It's a treat, it is shame you are missing out
If your manager decided to buy each of the team a Christmas gift of a box of Quality Street, your FT colleagues wouldn't feel upset if you got the same size box as them, rather than a box that was only 80% the size of theirs. They wouldnt complain to HR that the part-timers were being favoured, would they? If you happened to be on a diet when you were given the box of chocolates and so you couldnt enjoy it, you wouldnt give it back and ask for the cash so you could exchange it for some healthy apples, would you?