Yeah, I wouldn’t call it “the best week to get a mythic”, except for some weird criteria. Here’s a more wordy explanation combining what other people are saying.
Normally, when you try for a mythic, two rolls happen. First one roll decides what rarity of troop you will get. Then another roll chooses which troop of that rarity you get. But there are currently 34 mythics.
If you have 0 mythics, you probably want “any mythic” so that doesn’t matter. If you have 10-15 mythics, this gets problematic. There’s a 50% chance the mythic you get is one you already have. That gets worse every time you get a mythic.
This isn’t true during mythic week. You know exactly which mythic you’ll get. So if you want it, and try for it, you know you only have to win one of the two dice rolls. It kind of works against people too: once they have the mythic, if they don’t want duplicates it’s safest to stop spending keys.