C# - Communicating with an external class

Alright, what I am trying to do is make a program that when you put in the day of the month, like for example 1 would be January. It tells you how many days are in the month and what the month is, I need to make an external class called clsMonths in my solution explorer because thats what this assignment wants me to do.

So how do I communicate with this external class via my main form?

Im thinkin that I can create arrays in this external class and grab the values to use in my form.

Been trying to figure this out for a long time, been searching Google, and cant find much. I Couldn't figure this out on my own to save my life…

thanks for any help.