There are some things that you don't need to do every day, yet are still important enough to warrant a reminder. You know, like vacuuming, or checking your stock portfolio, or reminding yourself to tweet about how incredibly important it is to bring Firefly back to television. Google's got your back: at some point they added an "occasionally" option to Google Now's built-in reminders, triggered when you tell Now to "remind me" to do something.
The current options are for "today," "tomorrow," "occasionally," and then you can set a specific day and time. If you choose the occasional option, cards will show up at the top of your Google Now stack, and can be dismissed or postponed to another day. Exactly what interval Google will use isn't apparent to the user, though I'd guess it's somewhere in the range of two or three days. Utility for this feature seems a little limited - if I need to remember something only on demand, I tend to put it in Google Keep instead.
So, there you have it. One more way to tie a metaphorical string around your digital finger. Enjoy the functionality (it should be available to everyone), and thanks to Linus Metzler for pointing it out to us.