Learn French: Le printemps commence en mars.

In French, you generally must use a definite article (le, la, l', or les) when a season, day of the week, or general concept is the subject of the sentence. Since printemps is a masculine noun, it takes le.

Does printemps end in an "s" even when it's singular?

Yes, printemps always ends in an "s", whether it is singular or plural. The spelling doesn't change at all: un printemps (one spring) and deux printemps (two springs).

Is "in March" always translated as en mars?

Yes! When talking about an event happening in a specific month, you use the preposition en followed by the month. You could also say au mois de mars (in the month of March). You cannot use dans, which is mainly used for physical locations or a specific amount of time in the future.

Why isn't mars capitalized here?

Unlike in English, months of the year (like mars, avril, mai) and days of the week are not capitalized in French unless they appear at the very beginning of a sentence.