The last day to purchase into your NISA account for 2020 depends on what you are purchasing (and brokerages may vary about 1-2 business days, so I cannot speak for them all).
I use Rakuten Securities so I will post about them. I hope others may find this useful (or maybe it's obvious to the regulars here, in which case I apologize).
For Rakuten Securities
- today is the deadline for mutual funds (place your order before 15:00?)
- Dec 25 (by 8am) is the deadline for US stocks
- almost everything else can be done as late as Monday the 28th
- when purchasing mutual funds, you can see a 受渡日 which designates when the money will go into your account, just make sure it is in 2021