The Grammaring Guide to English Grammar with Exercises

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

Quotes:

Christmas is a time when you get homesick - even when you're home. - Carol Nelson

To be interested in the changing seasons is a happier state of mind than to be hopelessly in love with spring. - George Santayana

April is a promise that May is bound to keep. - Hal Borland

In summer, the song sings itself. - William Carlos Williams