If you pay in at the end of the year, you have to make sure that the deposit is credited to your 3a account in the same year. If the money does not reach your 3a provider until the beginning of the year, you may have a problem. This is because the 3a provider may not credit 3a deposits that were instructed too late back to the old year. And retrospective purchases into the pillar 3a are not possible (yet).
The value date of the payment is decisive for the correct allocation to the old or the new year. The same value date is used for both transacations – the debit on your private account and the credit on your 3a retirement savings foundation. If this date is in the new year, then your payment was too late.
However, it may be that the value date is in the old year, but the credit entry (booking date) was made in the new year. In this case, you can hope that it will still be possible to take the old year into account. However, we cannot speak for other 3a foundations, which is why it is best to get in touch directly with your 3a provider.
If you want to be sure, you should do your payments already before Christmas. This way you can still react if a mistake has happened and the payment, for whatever reason, does not reach the target.