The wording of the reminder found in 'Settings -> About ChromeOS -> Additional details' is:
> "This device will get automatic software and security updates until June 2024."
That leads (this) one to believe that it is the 1st of the month.
And the AUE page states:
> "Every Chrome device receives regular updates from Google until it reaches its Auto Update Expiration (AUE) date"
So it is in agreement with the statement in Settings.
I would be surprised if it's the last of the month or any day in between but if so, maybe the wording could be changed a tad to say so.