A wonderful or especially memorable day can be called a red-letter day. If your grandpa loves to fish, a red-letter day for him would be one he spends on a boat, reeling in one bass after another.
The day you win the Pulitzer Prize would qualify as a red-letter day — while for a preschooler, a red-letter day might be the day she gets to wear her Batman costume to the playground. The original red-letter day was a feast day or saint's day marked in red ink on church calendars. The tradition spread to academia and to many modern calendars as well.