Assuming you want to top-up $37,740 under the CPF Voluntary Contribution Scheme, the best time to top-up is at the end of the month since topping up at the beginning of the month will not earn any extra interest as interest is calculated based on the lowest balance for that month. Of course, in practice, you do not want to top-up on exactly on the last day of the month because you need to take into consideration of the turnaround time required for the computers and the cloud to sync and to run their batch processes at night - you could end up topping up on the 1st of the following month which is the worst-case scenario.
