Washington is going through a long stretch of anniversaries and commemorations, not all well-known, but each connected to the city in its own way, from Pi Day this past Thursday to the start of spring on March 20.

Thursday was Pi Day because its date, 3/14, suggests the value of pi, the ratio of a circle’s circumference to its diameter, especially meaningful in a city of so many circles.

Friday was March 15, which once was federal income tax day. Saturday’s date, March 16, is the birthday of President James Madison.

Sunday falls on the 17th, St. Patrick’s Day, so the city’s annual parade will be on the actual date, a rare occurrence.

Monday, the 18th, especially significant in a city of lawyers, is the 50th anniversary of the Supreme Court decision in Gideon v. Wainwright, expanding the right to counsel.

Ten years ago, on March 19, in this time zone, the Iraq war started. This year, the 20th starts astronomical spring, welcomed in Washington and beyond.