I was surprised to find how many words there are in English for what the Dutch just call an agenda: personal organizer, datebook, date log, daybook, day planner, personal analog assistant, personal planner, year planner, appointment book, appointment diary, calendar or agenda.

That'll help the search function. Now, feature request: it would be nice to have one on-wiki because it would be nice to have wiki-related matters in a planner on the wiki itself. It would better integrate with workflows than an external planner.

I imagine it would be something like this: an additional tab on Special:Preferences or an additional link in the section with links for your watchlist, contributions, logging out, etc. You would have a date picker, enter some custom text and notification method. At least Echo notification should be available. When the date comes, you get an echo notification with the message you entered. Being able to enter recurring events would also be appreciated, but even without that I believe this would be useful.

A few examples of useful reminders to set:

• Weekly reminder on Friday to translate Tech news (Special:TranslatorSignup doesn't support Echo without talk page/mail notification and doesn't allow signing up for just one thing)
• Reminder to close an RfC
• Reminder to tally up your score for a contest like WikiGap
• Reminder to check back with a copyright holder (in case they don't respond to your initial inquiry, you may otherwise forget to ask again)
• Reminder one week or one month into the future to check the uploads of a user who uploaded some copyvios to make sure they haven't uploaded any new copyvios
• Reminder to import some photos from Flickr that you can't import at that moment because either your work proxy blocks Flickr or your coffee break is almost over
• Reminder to sort out problems with an article that you don't have time to fix the moment you've spotted the issue

I think this could also increase engagement.