First open the post you want to edit. At the bottom left of the editor there is a "Post Options" link:

Click "Post Options" and the post date and time fields will appear in the bottom right corner of the editor:

Change the date to how you see fit and click "Publish Post". For example, if it's a special Christmas message, set the post date to 12/25/07 at 11:59 PM and your message will stay on the main page at the top of the posts until midnight Christmas day.