If it's emailed to you on 15th of every month, chances are it's generated on 15th 12AM so anything post that should technically be shifted to next cycle.
A lot of times, charges are stuck in pending state for transactions I made on 14th and weren't added to bill till the 15th and hence showed up in the next cycle. So, a lot fo factors would come into consideration like how fast the merchants processes the transaction, their payment processor and so on.
But the safest best would be to pay on 15th afternoon or evening to guarantee that the transaction is posted in the next cycle.