Prozbul

Our Mishnah begins like this.

Prozbol ha-muktam, if you predate a prozbol.

Now, most people write a prozbol on the last day of the Shemitah year, meaning the 29th day of Elul, and this way the prozbol would capture all the loans that may have been made over the last number of years, all the way up until the end of the year.

And that's because, for instance, if you were to write a prosbel on Rosh Chodesh Nisan of the Shemitah year, that's good.