SKY EVENT?
Someone suggested that the CME/Solar flare might have been the SKY EVENT, but that's not something that could have been predicted, in terms of timelines/markers.
The passing of a comet, however… That's something you could put on the calendar…
Or perhaps not - according to Wikipedia, is was only discovered discovered on March 27, 2020. Not early enough to have been used as a marker in a drop from May 2018… Unless it had been previously observed/discovered, but not publicly shared.
https://en.wikipedia.org/wiki/C/2020_F3_(NEOWISE)