scheduled site backup at specific time / hour
Try Elysia Cron, in addition to Backup and Migrate, you can set the time of day to run as well as many other things.

Don't forget to read the documentation.  

It is also worth noting that in the default configuration of Drupal 7 cron is triggered by users hitting your site.  If the site is not used exactly at the time indicated in Elysia Cron it will be run as soon as someone loads something on your site.  Therefore, it is impossible to get this to execute exactly when you want, but it should be close.