Go to Company setting, tab 13th
In this tab you can define the period in which the thirteenth salary will be paid for the employees who are entitled to it. You can define as many periods as there are months in the year. It will apply automatically to all entitled to 13th. If someone want another periodicity, it should be mentioned in his employee's card. (Not possible in case of separate 13th salary)
It is possible to pay the 13th salary separately but only in December. If the employees are paid monthly, it is possible to pay either the 13th or December first. However, if there are people paid by the hour, this is only possible if December is paid first. (So that the December hours are taken into account for the calculation of the 13th).
It can be paid with the November salary but only for employees paid on a monthly basis.
About withholding taxes, click here.