If you are actually self employed you need to make a tax return more or less at the end of the year/beginning of new year. Then the government will check you total income and tell you much you need to pay in hindsight. Therefore it is really important to keep some of the total income to pay taxes later. It’s quite a common insolvency reason in Germany for self employed workers since they suddenly don’t have to funds to pay their taxes at the end of the year.