red-letter

形容词 red-letter:

1. memorable for being a special occasion

red-let.ter

adj.（形容词）
Memorably happy:
值得纪念的喜庆的：
例句：
a red-letter day.
值得纪念的吉日

来源：
[From the practice of marking in red the holy days in church calendars]
[由用红色在教堂日历上标志出宗教节日的习俗而来]