The December VAT return in Italy is a special case compared to other countries. If you are VAT registered in Italy, usually you get the notification for the current month in the following month, for example you will have to pay the VAT for October in November.
The exception to this rule is December.
In December you will have to pay for November and also a prepayment for December. The calculations for December are made by estimating average vat paid in the past, then in January you will receive the correct amount for December and in case of difference you will either get credit money or pay the difference.
Given the following example:
Case 1: You receive the notification for prepayment of 100 EUR VAT for December 2018 and you pay it. In January 2019 you receive the correct VAT amount for December 2018 you have to pay, and this would be 120 EUR, then you only have to pay another 20 EUR.
Case 2: You have to prepay 100 EUR VAT for December 2018 and you pay it. In January 2019 you receive the correct VAT amount for December 2018 which would be 50 EUR. In this case, you have 50 EUR credit, which you could use to pay in February 2019 the VAT amount for January 2019.