If the employer invited you to interview for the position, there's no need to send a letter; you can directly confirm the meeting by phone or email. (work.chron.com)
But as of now, they won't even confirm a meeting occurred. (mauldineconomics.com)
Also, confirm the meeting location, how to get there, and set out anything else you'll need the following morning — like your keys. (theeverygirl.com)