I built one of these for myself in 1995 and have been happily using it ever since. Here's the source code:
0 0 19 7 * /usr/bin/mail -s "REMINDER: john T. birthday" me@mydomain.com
0 0 1 8 * /usr/bin/mail -s "REMINDER: MAKE xmas hotel reservations NOW for good pricing..." me@mydomain.com
This is why I like HN, and why it continues to be a much-higher-quality place to visit on the web than many other forums: there were no snarky replies, no "I can't believe you don't know cron" - just multiple friendly, helpful replies explaining exactly what this format is.
Agreed, though my first reaction was definitely: "I can't believe you don't know cron". I had to step back a little and remember who the audience is. Then I wrote my comment (at the same time as everyone else here did).
Actually my first thought was - that guy was a smart-ass showing off. Of course many people wouldn't know a crontab line - Windows (and I guess Mac - never had one) don't have such magical tools.
The file is a "cron schedule" [1]. Paste the series of numbers (and the *) into the box on the site below and you'll see it means "midnight on July 19th every year". The commenter will probably have this in their personal "crontab" file. They're emailing themselves every year on that date.