Learn Spanish (Latin America): Nuestra fiesta es en marzo o abril.

In Spanish, the months of the year are always written in lowercase unless they are the very first word of a sentence. This is also true for days of the week and seasons!

Do we always use en to say an event is "in" a specific month?

Yes! Just like in English, you use the preposition en (in) directly before the month to say when something happens, like en marzo (in March).

Why is it Nuestra fiesta and not Nuestro fiesta?

Possessive words like "our" have to match the gender of the thing being owned. Because fiesta (party) is a feminine noun, you must use the feminine form nuestra instead of the masculine nuestro.

Why use es instead of está for a party's date?

The verb ser (to be) is always used to tell when or where an event takes place. Even though we normally use estar to talk about where physical objects are located, events like a fiesta are an exception and always use ser (which becomes es for "it is").