When double time is required
Federal law never requires it. California does: after 12 hours in a workday, and after 8 hours on the seventh consecutive day of a workweek. Everywhere else, double time comes from a union contract or an employer's own policy, commonly for holidays or for a second consecutive shift.
Double time versus time and a half
At $20 an hour, time and a half is $30 and double time is $40, so every double time hour is worth $10 more than an overtime hour. On a 12-hour California day the first 8 are regular, the next 4 are 1.5x, and anything after that is 2x. The overtime calculator splits a whole week that way when you pick California.
Holiday double time
Many employers pay 2x for Thanksgiving, Christmas and similar days, but it's policy, not law. Use the holiday pay calculator with whatever multiplier your handbook lists.