I don't see the start of the tribulation occurring in a single event. The tribulation judgments are a marked period of increase of the tribulation we have faced in the world from the beginning. Now, while I can't pinpoint when the tribulation wrath starts, I do believe that when you see Jerusalem surrounded and being evacuated, don't bother to grab your toothbrush.
(Dan 9:27)
That Covenant will be for one week that the man of sin will enter into with others including Israel at the start of the one week (7 year tribulation). So when you see (if we are still here) such a Covenant signed with the man of sin and Israel and others, that triggers the great Tribulation.