5 Capitalization

5.6 Dates and periods

Capitalize the names of days, months, festivals, and holidays:

Tuesday

March

Easter

Good Friday

Ramadan

Passover

Thanksgiving

Christmas Eve

the Fifth of November

New Year’s Day

Names of the seasons are lower case, except where personified:
William went to Italy in the summer
O wild West Wind, thou breath of Autumn’s being

Capitalize historical periods and geological time scales:

the Devonian period

Early Minoan

the Upper Palaeozoic era

the Bronze Age

the Middle Ages (but the medieval period)

the Renaissance

the Dark Ages

Modern periods are more likely to be lower case; check such instances with the New Oxford Dictionary for Writers and Editors:

the space age
the age of steam
the jazz age
the belle époque

Use lower case for millennia, centuries, and decades (see also 11.6):

the first millennium

the sixteenth century

the sixties


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