Essentially Event is just some date (an optionally time) in Notes.
If you will type in the note something like this:
Check the site on [2018-01-21 -1d]
then SN will mark the note by Event tag (see above) and will start tracking the note for the time. On 2018-01-21 minus one day SN will start showing the note in active/pending events list (button on the top).
Any note may have such events, one or several.
I shall admit that implementation of the feature took one week more than I expected – I realized that it could be more types of such fields (I name them note activators) – not just Event. And so I needed to create infrastructure for them.