I remember being in a drama lesson, about a quarter to eleven in the morning, our teacher was late turning up for the class and when she finally arrived she told us the news that Thatcher had resigned. It's not quite the Kennedy assassination, but it's something I do quite vividly remember.
Definitely 28th November was the day she resigned as prime minister (i.e. went to Buckingham Palace and asked the Queen to appoint John Major insead); 22nd November was the day when she announced that she would resign as leader of the Conservative Party (and as Prime Minister), but did not actually do so until the leadership election had been organised and held.
Do you mean in office as Prime Minister? 1979-1990, Leader of the Conservative Party? 1975-1990 or just in office as a Minister or perhaps a Junior Minister? or even as an MP?
Oh by the way, she became Prime Minister on 4th May 1979, not the 3rd. The general election was 3rd May but she didn't go to Buckingham Palace until the next afternoon.