Red-Letter Day - a memorably happy or noteworthy day
It's a red letter day for Semantica. Memorable enough for our Chief to greet the readers with the monthly update :D
The expression, by the way, comes from Medieval church calendars in 1325 when saint's days, feasts, and other holy days, which came to be printed on church calendars in red. The term came into wider usage with the appearance in 1549 of the first Book of Common Prayer in which calendar showed special holy days in red ink.
Pic : Google Images