This question keeps coming up. So let's see if I can help to either clarify or further muddy the waters.
There are three kinds of funds in the world:
1.
Index funds, which (in the context of this discussion) track the S&P 500 index as exactly as possible. These funds have no discretion about what and how many shares they buy/sell of stocks in the index, and only a very little bit of discretion about the timing; when a new stock is added, or one dropped, or at the quarterly rebalancing, the company that owns the intellectual property of the index (
About Us | S&P Dow Jones Indices) tells them what they need to buy/sell, and gives them a window of 7 trading days (3 before, the day, and 3 after) around the official date to do the necessary trading. (TSLA is special; they've never before given this much notice, or even hinted at doing two phased adjustment periods. That's because TSLA is by far the biggest first-time entry.)
2. Funds which are
benchmarked against the S&P 500 index. I used to be a trustee of a $2B 401K plan, and every quarter, Fidelity (who managed it for us) would come in and show us all the various funds that members could invest in, like Vanguard funds, targeted retirement age funds, etc. All of these funds' performance would be charted against some "benchmark" so that we could judge whether the fund was being managed well, was fiscally responsible, and so on. Some (but by no means all) of these funds used the performance of the S&P 500 index as their comparison. But they had flexibility about whether or how much of individual stocks they could hold. I'll give an example:
ESGV is an ETF (exchange traded fund) that mirrors one that is often offered by pension/401k plans. It is basically the S&P 500 but with a focus on social and environmental responsibility, so it doesn't hold oil/gas stocks, gun stocks, and so on (and BTW slightly outperforms the S&P index). These funds obviously deviate from the exact S&P holdings, and exercise discretion about when to trade components, but they are still going to be compared to the S&P 500 when they are being judged, so they have to be somewhat conservative. They generally could maybe hold a little of a stock that wasn't in the S&P 500, but placing a big bet on such a stock would cause a big problem if it went down. Conversely, now that TSLA is being added to the index, they pretty much have to buy in, because if TSLA does well, and they aren't holding it, they will underperform the index. So they do have to buy eventually, but can choose when to do it. These are the guys running up the price now.
3. Funds that
don't care about the S&P 500 index. They will generally be benchmarked against something else. This includes funds like the ARK ETFs. Some of them will want to buy TSLA just because it will be viewed as a "safer" investment now than it was two weeks ago. Some were already buying TSLA. Some still won't (like the ones that track the oil&gas industry).
So, some of the #2 funds are already buying, some will be waiting, perhaps because of internal guidelines or slow decision making processes, and even the ones buying will be trying to do it as slowly as possible so as to not run up the market. The #1 funds can't buy yet; exactly when they can start will be determined by the index when they announce when/whether the tranches are happening, but the whole purchase window will close on the 24th, three days after the 21st, official inclusion date.
