A: On most of the letters there is a place at the top where you'll see the words start and end. Next to start you put the time you sent the letter and next to end you put the time the people who recive the letter have to send it by or it won't be effective. It should look something like this:
Start: 3:05 pm
End: 12:00 midnight
So, I would have sent the letter at 3:05 in the afternoon and whoever I sent it to has to send it by midnight if they wanted it to work.
Not all letters have this and of course you don't have to do it if you don't feel like it.