I am a government employee working for a non-appropriated fund activity in the Defense Department. I get paid by the hour.
My working hours are M-F 0730-1630, 40 hours per week. Sometimes I am made to work 8 hours or more on the weekend, but am never paid overtime. Instead I am given those hours off sometime during the following week. Can my employer legally manipulate my work days in order to avoid paying me overtime.

Your employer can set and change a work week that is different than the traditional Sunday through Saturday week. However, at the time a work week is changed, employers are to calculate overtime under both work week scenarios and give the employees the greater of the two numbers. That calculation is performed once - at the time of transitioning to the new work week.

Let me know if you have any other questions.