red-letter day
noun [ C usually singular ] uk/ˌredˈlet.ə ˌdeɪ/ us/ˌredˈlet̬.ɚ ˌdeɪ/
值得纪念的日子；有特殊意义的日子；喜庆日
a special, happy, and important day that you will always remember
The day I first set foot in America was a red-letter day for me. 首次踏上美国的那一天对我来说有着特殊意义。