Since no one else has bothered to say it, I’ll give you some excellent advice.
QUIT!
If you give two weeks’ notice and work them out, you’ll be doing more than most people in entry-level jobs. We had a bunch of people “quit” this summer by neglecting to show up anymore.
Quit with no guilt and move onwards and upwards.
Not only should you hand in your notice, but in the course of telling them why if they ask, DO NOT give them an opportunity to fix it this one time so that you will stay. Take the new job offer and don’t look back. If things go to shit again at the current job, you will then feel even more beholden to stay since they granted a favor and gave you what you wanted so that you wouldn’t quit now.
Get out.
Get out now and don’t let them drag you down!
You have a better place on the horizon. Seize it!
Besides, there is no such thing as a loyal corporate employer. No way, no how. No guilt, baby.
I agree with Jodi.
Look, you work for minimum wage. Your boss chooses to reward your loyalty and your seniority by giving you the shittiest shift possible.
Is your manager working those days? Know why not? BECAUSE THEY’RE FOR THE STUPID ONES!
QUIT! Even if you didn’t have the other job, you’d be better off telling them you’re quitting as of the 23rd. Enjoy Christmas, then start looking for another job immediately afterward.
Look at it this way: the courtesy you extend them obviously isn’t being reciprocated. You owe them nothing. I realized that companies are out to make money. Period. You work so you can live, right? Take the job that offers you the best opportunity for a happy life.
Screw 'em. Ditch the job. Take the new job that pays better and offers better hours. I can see how they’d want a more experienced senior staff member to be working on Christmas to better handle things, but if frankly if they aren’t going to give you the consideration you’re obviously giving them, you need to be thinking of #1 right now. You’ve a better job offer, and a chance to spend time with your family on Christmas. That right there is the closer as far as I’m concerned. You’ve paid your dues to the soulless Brand X company who do not seem to have regarded you with equal measure, so … bugger 'em. Give 'em your two weeks – more courtesy than they probably deserve if they’re anything like the last number of retail jobs I’ve worked – and move onwards and upwards. You and your family are worth more than a crummy low-wage job.
You are correct, however, if an employee asks for something simple, like not having to work nights on both days of the holiday, or something, it’s pretty dickish to force them to work exactly the schedule they requested to avoid.
I once worked at a pizza place during the holidays. I knew we’d be open Christmas eve, Christmas Day, and New Year’s eve. I told the boss that I’d like one of those days off, he could pick…I just wanted to be able to spend one of the holidays with my family or friends. He scheduled me for all of them anyhow and for the longest shift on each, even though there were 2 or 3 others who could have at least covered the long shift on one of those days. He was just an asshole, plain and simple. Sounds like Kytheria is experienceing something similar. I’d feel no worries about quitting on the spot. In fact, it’s how I (later) quit the job at the pizza place.
As Chris Rock said- What does it mean if your job pays you minimium wage? It means it they were allowed to pay you LESS, they would.
Something I haven’t seen mentioned (although I confess to skimming the posts, not a thorough reading) - when you turn in your two weeks notice, be prepared financially for them to terminate you on the spot. I’m not sure what your position is, but in some instances employers don’t want folks who are headed for the door to have the opportunity screw things up for the company in their final days.
Not saying that your employer would do this - just be prepared financially in case they do, and consider adjusting the length of your notice if you suspect this may be an issue.
Best of luck!
As Kythereia lives in Canada, and has been at the job for more than 3 months, it would be difficult for them to do this. Certainly they could just leave her off the schedule, but they wouldn’t be able to fire her without cause for giving notice. (Well, they could but they won’t - I’m VERY familiar with the company in question - they are anxious to avoid any sort of “disgruntled employee” type issues with the labour board.)
Excellent point - I hadn’t noticed that Kythereia is in Canada.
Carry on 
Sounds like it’s a unanimous QUIT and you’re ready to go for it. Let us know how it turns out.