It is allowed.
It is also a very bad thing to do, for many reasons. Good employers don't do things like this unless there is a real emergency in progress. With a real emergency in progress, a good employer will not hesitate to reimburse the employee for the lost money. It is only fair to the employee.
You do not say whether the "employer" revoked the holiday for everyone at the company, just for your workgroup, or just for you. This is a significant question. You do not say whether it was revoked, or just rescheduled. If your normal workweek is Tuesday through Saturday, then moving the holiday observation to Saturday is technically OK. Given that you made travel plans for a long weekend, I surmise that your normal workweek is Monday through Friday.
You do not say whether you are a member of a labor union or not. If you are represented by a labor union, your first stop is the shop steward. Odds are very good that a shop steward would be able to remind the employer that holiday schedules are contractual, and arbitrarily abrogating a contract provision is a very good way to cause serious difficulty with the union membership and leadership.
If you are not represented by a union, or if the shop steward gets nowhere, your next step is to go talk with the HR Director. Explain that you relied on the company's posted holiday announcement, made the travel plans in good faith, and have actually paid out money for the planned travel. You can then ASK for a compassionate exception to be made to the revised holiday schedule. If no exception can be made, you can ASK for the company to reimburse him for the nonrefundable portions of the travel money.
Next, you need to take a long, hard look at your situation, and consider the possibility that your employer might be doing you a favor by firing you under these circumstances.
Finally, state employment commissions usually take a very dim view of this kind of thing. My read is that the odds would be VERY good that a firing under these circumstances would be ruled to be unjustified and unjustifiable, and result in the employer's unemployment insurance rates spiking up significantly.