The zero article with names of days, months, seasons, holidays and parts of the day

Names of days, months, seasons and holidays are normally not preceded by an article:

on Monday
in March
in summer
at Christmas

The same is true for certain parts of the day (with the exception of in the morning/afternoon/evening):

at noon
at night
at midnight
before dusk
after sunset
at sunrise

But if there is an adjective before the noun or a phrase or clause after it, the noun usually takes an article:

on the Monday of that week
the summer I met my future husband
the best Christmas I've ever had
it was a beautiful sunset