I would guess it's at the end of every month!

It's at the end/start of the every month.

Ok gotcha. So basically if I came into Steam at the middle of the month, it would make sense to say the first payment probably wouldn't come in until the end of the following month?

Something like that.

Cool deal! Thank you all for your help!

You get your months sales at the end of the following month. So, as an example, all sales in May would be paid to you at the end of June or thereabouts.