Today is just the big picture. We'll go into each type over the next two days.
An Event is a challenge you choose to join (usually 7 days). Stream as usual while it runs, and your streams count. When it ends, you get Prizes based on how far you got.
Two types
What differs is how you earn your Prize.
Achievement: reach the goal and it's yours
Each Event sets its own goal, such as Event Points, Stars, or streaming days. Below is an Event Points example. You aren't compared with anyone else, so you can aim for your goal at your own pace.

Event Ranking: your place decides your Prize
Everyone who joined competes on their total Event Points.

When Events start and end
Most Events start Tuesday night and run for 7 days.
Time | Summer | Winter |
Start (Tuesday night) | 7:00 PM ET | 6:00 PM ET |
End (Monday night, 7 days later) | 11:59 PM ET | 10:59 PM ET |
There are exceptions.
Some Events start on other days or run longer or shorter. Always check the start and end times on the Event page. Times in your time zone: Event and Date Rollover Times in IRIAM