What day would be the least busy at Six Flags, the 26th, 27th, 28th, 29th, and 30th of December?

I don't know what the rules against linking other websites are but here

http://www.isitpacked.com/crowd-calendars/six-flags-magic-mountain/

You could have just done a quick google search

I did find that page after I made the post, I guess a better question would have been which one of those days would have been the best.

The tricky thing is that SFMM can be very unpredictable during winter break. I've gone during that period and found the longest line in the park was 5 minutes, and I've also gone and had the shortest line be 80. I myself am planning to go on either the 27th or 28th with family visiting from out of state, and I've been telling them to expect 90 minute waits for headliners (though they may be less). As a general rule, the earlier in the week you go the less crowded it will be, but weather and special events can sometimes mess with that. If this is likely your only visit to the park for the next several years, I recommend budgeting for a Flash Pass in case it is necessary. Otherwise, have everyone in your group pick their top three must-do coasters and prioritize those, skipping other rides until you've done everything on your priority list.

Ah, thank you. I actually have a membership, but have only used it twice in the last four months. Good thing it's good for as long as I want :)