Harry receives many letters unexpectedly at breakfast and realizes that the Quibbler interview he gave to Rita Skeeter on Valentines’ Day has been published in the March issue.
Umbridge is not impressed and tries to ban the magazine. By mid-morning, Educational Decree #27 makes possession of the Quibbler an expulsion offense. This has the exact opposite effect of what she’s hoping for. Everyone now wants to read it (OP26).
The magazine is published on a Monday immediately after the Gryffindor's loss to Hufflepuff. That match was on February 21, so the Quibbler is published that weekend and turns up on the breakfast table in the Great Hall on Monday the 23rd.