cron & crontab:

cron is a Unix command for scheduling one or more commands (collectively called a cron script or cron job) to be automatically executed on the computer at a specified time(s), usually periodically, such as emailing daily reports.  Cron is a daemon process, which means it runs continuously in the background, waiting to run a specified operation at predefined times or when specific events occur.  crontab is a Unix command that creates or modifies a file (called a crontab file) that contains a cron table (cron tab) or list of Unix shell commands, each having a specified time of execution by the operating system.

Here is a list of free web hosting sites allowing cron jobs.