As a member of the Order of the Phoenix, Mr. Weasley takes a turn standing guard at the Department of Mysteries. Harry sees the snake attack in a dream (OP21). Harry brings his vision to Dumbledore’s attention, and other members of the Order rush to save Mr. Weasley and get him to St. Mungo’s. Harry, Ron, Fred, George, and Ginny, are transported to Grimmauld Place. (OP22)
Chapter 21 begins on the Sunday in November immediately following the Gryffindor-Slytherin match and continues through to the evening of the final D.A. meeting. We know it's a Wednesday because Hermione says two days later that it's the last day of term, so with the calendar we've been using for the term, that would make this the night of 18 December.