Help with understanding half-price upgrades...

[Bittersweet92101]

Or any reduced price upgrades actually. I'm seeing these from time to time, but I don't see any mention of them in the events information. Just a couple of hours ago, I was seeing half price costs to upgrade some of my buildings (it may have been all of them, not sure). I'm just wondering why? Is it part of the event and I missed that in the event information? Is it something that happens on a certain time cycle? Is it something random that just happens unannounced from time to time? Thank you in advance for any help.