The advantage is to maximize tax relief based on 2021 income (for Year of Assessment 2022). First of all, do you have a December, 2020, or January, 2021, payroll cycle of compulsory contributions that'll hit sometime this month (January, 2021)? Do you have any MediSave withdrawals that'll hit this month, such as an Integrated Shield, MediShield Life, or CareShield Life premium? If so, and if you want to maximize tax relief and interest, you should try to beat those events by a couple days.
Let's suppose for example your December, 2020, payroll cycle contributions usually hit on the 10th day of the month and you have an Integrated Shield premium payment coming out of MediSave on January 25. Here's what I would do:
1. Boost your MediSave Account to $63,000 and your Special Account to $186,000 on January 7. This assumes you expect to have enough room below the CPF Annual Limit for your MediSave voluntary contribution.
2. On January 30 or 31, fill up your MediSave Account again to $63,000, again assuming you have enough room in 2021 below the CPF Annual Limit.
My understanding is that the CPF Annual Limit includes compulsory contributions based on how they're "tagged," not based on when they're credited. So your December, 2019, payroll cycle (that was credited in January, 2020) counts toward the 2019 CPF Annual Limit, not the 2020 CPF Annual Limit.