How can I tell when events end?

Solution 1:

You can also find this information in the Arena log file. This might be more reliable as the Event calendar has not always been kept up-to-date. The log file is called Player.log and can be found in the folder %appdata%..\LocalLow\Wizards Of The Coast\MTGA\.
(which expands to C:\Users\<CurrentWindowsUser>\AppData\LocalLow\Wizards Of The Coast\MTGA\ with <CurrentWindows> replaced with the name of the current Windows user)

This works with MTGA 2021.8.50 (current version as of now), but might break with future updates as WotC quite often changes things around in the log file.

You might have to enable Detailed Logs in the game's options. It's under Adjust Options (gear symbol) > Account > Detailed Logs (Plugin Support).

Search for the Event name, e.g. PremierDraft, and you will find the end of the event in the JSON field ClosedTime.

{
  "InternalEventName": "PremierDraft_AFR_20210708",
  "EventState": "Active",
  "EventType": "Limited",
  "StartTime": "2021-07-08T15:00:00Z",
  "LockedTime": "2021-09-16T15:00:00Z",
  "ClosedTime": "2021-09-16T18:00:00Z",
  [...]
}

The timer starts with 48 h left from my experience.

Solution 2:

You can do so by consulting the event calendar and noting it down.

On the event calendar you will always see ongoing events and their end-date. Sometimes it will give you exact dates like "10pm UTC," and sometimes only the start and end date.