Learn Swedish: Vår mamma besöker oss i april eller maj.

The word for "our" changes depending on the gender of the noun. Because mamma is an en-word, we use vår. If it were an ett-word like barn (child), we would use vårt, and for plural nouns we use våra.

Why doesn't mamma have the definite ending, like vår mamman?

In Swedish, when you put a possessive word like vår (our) or min (my) in front of a noun, the noun stays in its basic, indefinite form. So it is just vår mamma.

Is oss the Swedish word for "us"?

Yes! Just like "we" becomes "us" in English when it receives the action, the Swedish subject pronoun vi (we) becomes the object pronoun oss (us) here because we are the ones being visited.

Why aren't the months april and maj capitalized?

Unlike in English, months, days of the week, and languages are not capitalized in Swedish. They only get a capital letter if they appear at the very beginning of a sentence.