Index Reconstitution Calendars and Effective Dates

Index providers don't add or remove constituents whenever they feel like it — most run on a published calendar of review dates (commonly quarterly, sometimes annual for smaller indices), and every change goes through two distinct dates: the announcement date, when the provider publicly states what is changing, and the effective date, when the change actually takes hold in the live index. The gap between them — usually a few trading days to a couple of weeks — is deliberate: it gives every fund tracking the index time to prepare its trades.

That gap is also the reason index changes move prices. Passive funds tracking the index are contractually obligated to hold what the index holds, so on the effective date — specifically at the close of trading, since that's the price the index itself uses — every index fund needs to simultaneously buy the newly added stock and sell whatever was removed. A large enough flow of buying concentrated into a single moment at the close can push the stock's price up in the days leading into the effective date, an effect commonly called the index inclusion effect.

Worked example

A mid-cap stock is announced for addition to a widely tracked large-cap index on a Wednesday, with the change effective at the close of trading the following Monday — five trading days later.

Date	Event
Wednesday (announcement)	Provider publishes the addition; stock begins to react
Thu–Fri	Active traders and some funds front-run the anticipated index-fund buying
Monday, 3:00–4:00 pm	Index funds execute large buy orders into the closing auction
Monday, close	Change becomes effective; index level now includes the new constituent

By the time index funds actually transact at Monday's close, much of the price impact may already be reflected in the stock — some studies find prices partially revert in the weeks after the effective date, as the buying that pushed the price up during the announcement window was not driven by any change in the company's fundamentals.

Reconstitution calendars themselves are published well in advance — a quarterly review index typically fixes its review dates a year or more ahead — so the uncertainty in any given cycle is almost entirely about which names will be added or dropped, not when the change will occur. That predictability is precisely what lets event-driven strategies position around the announcement rather than only reacting to it.