Here are a few services that you can use to schedule your tweets.
From the name itself works like future email or future mail. It allows you to schedule your tweets to be published on the day/time you choose.
FutureTweets.com is a free service that lets you schedule your Twitter messages. Send it at a specific time in the future or send a reoccuring Tweet daily, weekly, monthly or yearly!
You’ll never forget the birthdays of your beloved Twitter friends again! Just schedule a nice Tweet on their birthday!
Keep your Twitter stream ticking over with new tweets even when you’re not in front of your computer. Or, use it as your personal reminder system.PLUS… Send automated thank you notes to new followers, and automatically follow new followers, if you choose to do so.
Twuffer another Twitter scheduler that allows the Twitter user to compose a list of future tweets, and schedule their release. A Twitter buffer.