In the past, there was a so-called "month of repentance" in some federal states, i.e. church tax liability ended one month after the month of resignation. This applied to Berlin, Brandenburg, Bremen, Hamburg, Hesse, Mecklenburg-Western Pomerania, Saxony, Schleswig-Holstein, Thuringia.
However, the month of repentance has now been abolished to standardise church tax regulations across Germany, i.e. the resignation from the church becomes effective in the calendar month in which it was declared.
In Berlin, Brandenburg and Bremen, resignation is free of charge. In the other federal states, you have to pay between 10 and 60 Euro for the certificate of resignation from the church.