Update to my original post about when the TSLA share price is actually being used to calculate the index value. This is the original press release about the addition:
Index News Dec 11, 2020 - 5:15 PMTesla Set to Join S&P 500 & 100; Apartment Income REIT to Join S&P MidCap 400
This is the original press release from S&P. My reading is that the entry price to the index is based on the Opening Price on Monday; not the Closing Price on Friday.
These are clearly very similar to each other, most of the time. In this circumstance, there may be a reasonably large difference
If that were true, the emphasis on buying shares would have been placed on Monday's opening cross, not Friday's closing cross. Otherwise index funds would be opening themselves up to huge dispersion risk. I think in this case, 12/21 is simply referring to the date that the index will make the listed changes for the purposes of displaying a value.