Monthly data fees are typically billed on the 1st every month up front. However let's say you enabled your data the prior month after the 1st so therefore you missed the up front billing cycle.
So generally what will happen is we will bill you in the arrears for that prior month usually around the 1st week of the current month, but you will also see the normal 1st of the month charge for the current month as well. So for example if you activated your data for the first time on June 15th prior month, you will be billed in the first week of July in the arrears for June, and then you will see also the July billing on July 1st as well.
You can also look at the description of the charges on your statement and you will notice it will specifically tell you what month you are being charged for. After you are caught up on billing then you will just see the standard one time charge every month which is usually on the 1st of each month up front.