This probably isn't too strategy related, but I am collecting information on planetary events.
Right now my hypothesis is that you get them when you get your hourly shipment.

So for in my case, three times in one week (Probably would've been 4, but who knows, because I wasn't on yesterday) I got a planetary event at my 3:15PM resource shipment, and to help confirm my hypothesis, someone in my battle group stated that they got a planetary event right as they got one of their shipments as well.

If you are offline at the time though, it occurs when you log on.

So far all of my random events (not colonization or invasion events) have happened when my upkeep timer hit 0, or the very next time i've accessed the game after upkeep timer hit 0
(which yes, cooresponds to ONE of your 24 hourly shipments)

After you said that, I checked my upkeep timer, and it also corresponded with when I get planetary events.

I would have to say this is correct. I was online when one poped up and I did indeed get a shipment. If it was at the same time I cant be 100% sure. But I would say it is a correct assumption.

there have been a few people... *glances at DarthFlagitious*
...who claim that they've had a few random planetary events that happened at a weird offset away from their upkeep time, and they weren't offline or idle during their upkeep time....

...but i haven't seen proof of such weirdness on my own account so i have to say they were probably idle and occupied with something/someone else at the time and really weren't paying attn

i look forward to the exact time my upkeep hits everyday (warning... don't ever share what time that is, or people will know exactly when to farm you for raids and hacks)...

Well it makes sense, you wouldn't find out about the things until you collected the resources from the planet.

The odd part about this is that whenever I'm offline when my upkeep is taken, I don't get an event, but when I'm online I do.

(As I don't get one at all and I always get one, each statement is respective to the top statements)