It's better to send Thank You letter to everyone who interviewed you, in 24 hours. The purpose is to impress people, and hopefully

helps on their writing a better review on you--that's why sending the letter early the next day is good (when most interviewees send back

their review forms usually).

You can simply write 3-4 sentenses, mentioning some of the things you have talked during the interview.

The content is not very important--no one is going to judge upon 3 sentenses. Just do it to show that

you are a polite person and appreciate others, a type of co-worker everyone prefers to work with.