Hello,
I work at a California tech firm. They recently went through a "regime change" in our department (helpdesk). Now for the first time, they are are not compensating those required to man the helpdesk on President's day by paying for the time worked that day (plus holiday pay) but are requiring us to take a day off during the week with pay. Before I did not mind working on Holidays but now I certainly do. Is what they are trying to pull legal? If not, can you direct me to California legal code that says that it is not legal?
Thank you.

It is legal. There are only three states, all in New England, where employers are required to pay a premium for working on any holidays. They are only required to pay straight time for working on a holiday; they are only required to pay overtime if you work over 40 hours in a week (or, in your state, over 8 in a day) and they may require you to take a day off during the week at any time, holidays or not, unless you have a bona fide, legally binding contract or CBA that specifically says otherwise.