I presume you're referring to the passage in the Clouds where Phidippides refers to the day of the old and the new (τὴν ἕνην τε καὶ νέαν)? A slightly more literal, though kind of nonsensical translation, for τὴν ἕνην τε καὶ νέαν would be "the day of last month and the new month." The ἕνη καὶ νέα is the last day of the month, by which time monthly interest on loans was supposed to have been paid. The ἕνη καὶ νέα was actually a period of two days, consisting of the last day of the old month and the first day of the new month, rather than a single day, which is where the pseudo-sophistic joke of the passage comes from

I am not able to comment on all of Classical Greek lending practices, but I am able to speak on some of the Athenian practices. Specifically before the Solonian reforms, individuals who went too far into debt could fall into Slavery. Professor Donald Kagan theorized it was likely that farmers took out loans to pay for the necessary items to plant their crops. If there was a poor harvest, they would fall into debt. He thought it was likely that a series of bad harvests lead to widespread enslavement of the population. This seems to be supported, as the great sage Solon was appointed to create laws to ease the pressures of the time (~6th century BC). One of the greatest challenges he faced was the massive outcry over the number of enslaved Athenians. The poor wanted all debt to be abolished, the lenders obviously were opposed to this. In line with his general style, Solon proposed a comprise: debts were not erased, but debt could not longer lead to slavery and many individuals who fell into slavery had their freedom purchased from the purse of Athena (form the state).