Prime Minister Theresa May has vowed for months that the country will trigger Article 50 by the end of March. But Monday's announcement of the date - March 29 - was the first official confirmation of the government's exact timing.
Britain's Parliament last week gave its final approval to May's Brexit plans, and the prime minister had at one point been expected to trigger Article 50 then.
Scottish leader Nicola Sturgeon threw a wrench in those plans last Monday by announcing a push for a new independence referendum from the United Kingdom, which also includes England, Wales and Northern Ireland. The Scottish move seemed to catch Downing Street off-guard, and may have contributed to a decision to push Article 50 notification back to the final week of March.
The prime minister heads into the EU negotiations with her premiership, Britain's economy and even the United Kingdom's viability as a unified country all on the line. May came to power soon after last June's Brexit referendum, and she has repeatedly said in the months since that she will deliver on voters' narrow decision to make Britain the first country to leave the European Union.
On Monday, she departed on the first stage of a "listening tour" that will take her across Britain in the lead-up to the March 29 move. Her first stop was Wales, and she was also expected to visit sites in Scotland, Northern Ireland and England in the coming days.
Although Britain as a whole voted 52-to-48 in favor of leaving, majorities of voters in both Scotland and Northern Ireland favored staying in the European Union. Sturgeon has charged that Scottish voters are being taken out of the European Union against their will, and said last week she wants an independence referendum - a rerun of a September 2014 vote that opted to stay in the United Kingdom - sometime between the autumn of 2018 and the spring of 2019.
May has sharply criticized that call, and said over the weekend that "now is not the time" for a Scottish vote. But she has not threatened to veto another referendum altogether.
Britain's exit negotiations are expected to be exceptionally tricky, with the country aiming to leave Europe's common market and customs union but hoping to retain preferential access to both through a new trading agreement.
May has signaled she will prioritize Britain's ability to control immigration from EU countries, a critical element in driving pro-Brexit sentiment. European leaders have drawn a tough line, signaling they will not allow Britain to enjoy the benefits of EU membership but not bear the responsibilities.
Once Britain has delivered its Article 50 letter to European Council President Donald Tusk in Brussels, EU leaders are expected to reply with their own letter setting out the bloc's negotiating stance.
If Britain and the rest of the EU fail to agree to terms by the spring of 2019, they will either have to extend the negotiations or Britain will simply fall out of the European Union without an agreement for its future relations with its biggest trading partner - a scenario known as "dirty Brexit."
May is hoping she will be able to run for reelection in the spring of 2020 on a platform of having delivered on the public's will. But economists and government officials have warned that Britain's exit is likely to be turbulent, and some within the prime minister's ruling Conservative Party have pushed for her to call an early election this spring.
The call would take advantage of polls showing May's Tories well ahead of the opposition Labour Party, which has been beset by internal strife under left-wing leader Jeremy Corbyn. May only has a narrow majority in the House of Commons, and a vote this spring would likely allow her to significantly broaden it.
But May has repeatedly ruled out an early vote, and on Monday her spokesman told British journalists that there was "not going to be one."