Plugin: WP-Cron 
WP-Cron is a plugin to support periodic execution of actions / plugins / stuff! From the readme:
WP-Cron provides a rudimentary support for scheduled exection of actions; a sort of “delayed action” processing for WordPress. It is nowhere near as robust as the actual UNIX cron facility, but should be good enough to “do stuff” on a fairly regular basis.
What with human beings, search engine spiders, and content aggregators, we can be fairly sure that even the most uninteresting (public) blog will be visited with some regularity. WP-Cron relies on this regularity to schedule the execution of three new plugin hooks. These hooks execute roughly once every fifteen minutes, every hour, or every day.
Included in the download are three example plugins to demonstrate how to use WP-Cron:
- wp-cron-dashboard: uses WP-Cron to update the data in your WordPress dashboard (roughly) every hour.
- wp-cron-reminder: sends out a generic reminder email to the blog admin (roughly) every 15 and 60 minutes. You can use this to prove to yourself that WP-Cron is working.
- wp-cron-future-pings: suppresses the default ping and trackback action when you post a new item dated in the future. Then (roughly) every 15 minutes the plugin checks to see if any future-dated posts are now visible to readers. If they are, the pings and trackbacks are sent! (note: wp-cron-future-pings requires WordPress 1.5.1.1 to work properly: see changeset 2604)
- wp-cron-mail: a plugin version of the
wp-mail.phpscript, written to use WP-Cron to support blog-by-email.
Download WP-Cron 1.1a!
Note: I dropped the revision on this version back to 1.1a, in order to make it easier for me to manage version numbers at the WordPress Plugin repository.
Follow developments of WP-Cron here!
wp-cron-mailexecutes roughly every fifteen minutes. If you'd like to change this to run roughly once per hour, change line 14 from:add_action('wp_cron_15', 'wp_cron_mail');toadd_action('wp_cron_hourly', 'wp_cron_mail');If you only want once per day, change hourly to daily in the line above.cronis a UNIX utility for executing scheduled tasks. The links in the blog by email instructions take you to information about the UNIX cron command. My WP-Cron plugin is a cron-like system, which operates strictly in WordPress. If you've installed the WP-Cron plugin, then you're done. Every 15 or twenty minutes, view your site. WP-Cron will occasionally be triggered, and your email submissions should be posted. Let me know if it doesn't work for you!add_action()is defined in /wp-includes/functions.php. It might be worth re-uploading the file, to make sure it's intact.add_action()is a standard component of most plugins, so I can't imagine why this isn't working for you.wp-mail.php, adjusted to supported WP-Cron. If you need to do blog-by-email, you might have better success with postie.