I work for a company that make you work a 48 hour week and pays straight time for all of the 48 hours?

today is the 8th of dec which is sat we work a full day today making it a 48 hr work week but will only be paid for 40 hrs the extra day will be moved to the 24th of dec to fill in for christmas eve where we will be off and still at straight time is this legal to do.

Call your local employment office; they will have your answer right away. (Different states have different laws, and there are Federal mandates, too).

I dont think its legal. but, you have to work the day before a holiday and the work the day after a holiday in order to be paid for it. So, technically, they dont have to pay you for christmas, if you dont work the day before (24th).
Employers dont really follow the rules all of the time,

Wage laws govern payment of wages. If you are an hourly, non-exempt employee you are legally entitled to overtime pay following 40 hours of work. This also applys to salaried, non exempt workers.

Exempt workers- managers, professionals are not governed by these laws and are not eligible for overtime pay. So the change is working hours does not impact their compensation.

Call your states employment divison for details.

No,as long as those extra eight hours are part of your pay week,your boss must pay time and a half over the forty.
Go in person to Department of Labor - Wage and Hour Division and ask to speak to an investigator. Make sure you take all info pertaining to this and fill them in on all details.