Event calendars are usually announced around the first of the month. The events run in order, although not necessarily one after the other.
The main thing to remember is that there are going to be times where there are two, three or maybe even four events going on at once, so when an event opens up it's best to get to work on the challenges. Nothing feels worse than procrastinating on one and then having two more pile up that make it seem like a grind to get them all done. That said, don't feel bad if you miss finishing an event.
The only reason to attempt to complete every event is if you are an absolute collector and must have everything. Perhaps it is necessary for those that are ultra competitive and want to have all the latest toys and gadgets as well. But the casual player should just focus on those events that give rewards that will benefit their current game.
A quick look at the timeline should show you when the next fragment event will be around, when the next sacrifice event will occur, and maybe even when something new and interesting might unfold.
Just keep in mind that they launch in the order listed, are spread through the month, and probably spill over into the next month. Those listed at the end will be during the final week of the month, while those at the top are going to occur early.
Two important things to note are :