For some MPs, 29 March will become a new independence day to be celebrated through the ages.
For others, it will be the moment that the referendum process ends, and reality begins.
Indeed, for most close observers of the EU Treaty, it had been commonly presumed that the moment of the trigger is most notable for the transfer of negotiating power from the exiting nation to the remaining EU. That theory will now be tested.
The clearest consequence of 29 March is that it sets the clock counting down to just before midnight on 29 March 2019.
Few people, beyond the PM and some Cabinet ministers, say they believe a comprehensive free trade partnership can be negotiated in that two-year framework.
A month of that time will be used up by the fact the trigger comes next week rather than last week: the EU will now not meet at summit level to consider Brexit until the end of April at the earliest and possibly mid-May.
One senior source told me that “notification on 29 March does not leave sufficient time to prepare a European Council on 6-7 April to adopt Brexit guidelines”, as had originally been pencilled in.
Once the letter is received next Wednesday, the EU 27 want between four and six weeks to consult with member states.
Of greater import is perhaps the mood music.
Judging by the European Scrutiny Committee, that might go in some unexpected directions.
Sir Bill Cash raised the fact that the UK wrote off half of Germany’s First World War debts during the 1953 London Debt Conference, suggesting that, therefore, “we owe the EU nothing” in terms of the so-called divorce bill of up to £50bn.
Meanwhile, in Brussels the gnomic and elusive Michel Barnier, who seems to use his Twitter account to send Eric Cantona-style subliminal messages, sent a picture of his fourth seminar on technical preparations.
The EU 27 has to “start preparing now” for future customs controls with Britain, he said.
Both of these suggestions point in the direction of a No Deal Brexit.
It is important to remember one fact: one of the initial intentions stated by the PM of announcing the March date for Article 50 in October was to start pre-negotiations.
They did not happen.
That is why, as the date was confirmed, the PM told the country to prepare for a “hard negotiation”.