Which digital calendar do you use? At this point I use Google's calendar. But like anything else that's digital I gotta have a backup in place. I went to Google's calendar forum and saw two things I did not like:
1) people complaining about the Google calendar all of a sudden failing. The hideous result to that is people then lose track of their future appointments, phone numbers, important dates, etc, etc...
2) none of Google's people were helping them fix the problem (that's typical of a Google forum). That's so wrong. Google has a TON of money.
Imagine your digital calendar failing. Imagine losing all your important dates. The horror! I need a backup calendar that's just like Google's: one that will send the user an email alert. Which digital calendar do you use?

http://www.google.com/support/forum/p/Calendar?hl=en

I like the calendar in Windows Live Mail:
http://download.live.com/wlmail

@ Cheery

I actually just use the little calendar in my phone and.or the calendar on the wall.

And when I bother to get one, I use a daily planner notebook

I find the Firefox add-on ReminderFox calendar a helpful little tool to remind me of very important events and birthdays.