Do you have to purchase a streak freeze before you skip a day, or can you apply it retroactively?
Do you get to pick the day or do you have to login the day you can't study and apply it then?
It hangs out in your account waiting to be used. It does not kick in retroactively, nor do you have to buy it the day you want to use it.
If you buy it on Monday with a streak of 5 and don't sign in on Tuesday, you'll log in on Wednesday to see a streak of 5 and no freeze (instead, you'll see the button to buy it again).