Monday 21 December is when the results of the December index review take effect, and for funds that mirror the FTSE 100, it is the day any reshuffle has to be reflected in their portfolios.

The quiet machinery behind the index

Most market attention this week is on the Bank of England, the US midterms and a heavy run of company results. But for passive funds that track the FTSE 100, a far less glamorous date is already on the calendar: Monday 21 December 2026.

That is when the changes from the December quarterly index review come into force, at the market open. The process runs to a fixed timetable. The Market Capitalisation data cut-off falls on Tuesday 1 December. The review results are announced on Wednesday 2 December, after the UK market closes. Any additions, deletions and adjustments then become effective from the start of trading on 21 December.

The FTSE 100 is weighted by market capitalisation, meaning larger companies count for more in the index than smaller ones. Heavyweights such as Shell (LSE:SHEL) and BP (LSE:BP.) move the index more than the smaller members near the bottom of the table. When a company joins or leaves, or when weights shift, funds designed to replicate the index need to adjust their holdings so that they continue to match it.

Lessons from September

The last review offers a useful template. In September 2026, easyJet (LSE:EZJ) and Ithaca Energy (LSE:ITH) were promoted into the FTSE 100, replacing Entain and Persimmon. Those changes took effect at the start of trading on Monday 21 September 2026.

Ithaca's experience since then underlines how quickly fortunes can change for newer, smaller members. On Monday 2 November, it was the FTSE 100's biggest faller, down 5.94%, as Brent Crude fell to $105.09 a barrel at the London close from $117.61 on Friday. Energy stocks bore the brunt of the oil slide, with BP down 4.37% and Shell down 2.86%, while the FTSE 350 energy sector fell 3.3%.

Persimmon's departure, meanwhile, is a reminder that even well-known names can drop out of the index. Barratt Redrow (LSE:BTRW) remains in the index and rose 3.5% on Monday as construction and materials stocks rallied 2.8%. Reports tied that broad rally to lower oil prices.

This article does not speculate about which companies might be added or removed in December. The outcome depends on market values on 1 December.

Why the cut-off matters more than the effective date

For anyone following the review, the sequence matters. The 1 December cut-off is the moment that decides the outcome, because it fixes the market capitalisations that the index provider uses. The 2 December announcement reveals the result. The 21 December effective date is when tracker funds must have their portfolios aligned.

That gap of almost three weeks between announcement and implementation is significant. It gives fund managers time to plan their trades, and it gives other Market Participants time to position around expected flows. Price moves in affected stocks can therefore happen well before the changes formally take effect.

Market moves between now and 1 December could still reshape the picture. The FTSE 100 closed Monday at 10,761.33, up 31.55 points, or 0.3%, after a fourth straight gain. Its 2026 range runs from a low of 9,670.46 to a high of 10,989.45. Big swings in individual stocks — like Monday's 4.76% jump in JD Sports Fashion (LSE:JD.) or Ithaca's 5.94% slide — show how quickly relative market values can shift.

This week's events could also move valuations. The Bank of England is widely expected to raise Bank Rate from 3.75% to 4% on Thursday 5 November, with markets pricing about a 95% probability. Company updates from Marks & Spencer (LSE:MKS), BT Group (LSE:BT.A), National Grid (LSE:NG.), Next (LSE:NXT) and IAG (LSE:IAG), among others, are due this week too.

Illustration: imagine a hypothetical tracker fund with £10bn invested in the FTSE 100. If a company were added to the index with a weight of 0.2%, the fund would need to hold about £20m of its shares. A company leaving the index with a 0.15% weight would require the fund to sell about £15m. Multiply that across every fund tracking the index and the potential flows become meaningful. The weights and fund size are invented purely to show the arithmetic.

The same logic applies to existing constituents whose weights change in a review, although these adjustments are generally smaller than full additions or deletions.

For ordinary holders of a FTSE 100 tracker, none of this requires any action. The fund does the work behind the scenes. But the timing explains why the date matters to the wider market: concentrated buying and selling in a handful of stocks, all aimed at the same opening session, can create unusual volumes and occasional price swings in the affected names around the effective date.

There is a coincidence on the calendar as well. Shell's quarterly Dividend of 39.06 US cents per share, which goes ex-dividend on 12 November, is due to be paid on 21 December — the same day the review takes effect. Index Funds holding Shell will be dealing with both the dividend income and any Rebalancing on one day.

Key dates to watch

  • Thursday 12 November: Shell ex-dividend date (39.06 US cents)
  • Tuesday 1 December: Market capitalisation data cut-off for the December review
  • Wednesday 2 December: December review announcement, after the UK close
  • Monday 21 December: December review changes take effect at the market open
  • Monday 21 December: Shell dividend payment

The bottom line

Monday 21 December may not grab headlines, but it is the day the FTSE 100's composition is formally reset for the new year, and tracker funds must be ready. The real decision point comes earlier, at the 1 December data cut-off, with the verdict published the following evening. Between now and then, oil prices, the Bank of England and a busy results season could all reshape relative market values. Investors will be watching the 2 December announcement to see whether the September reshuffle that brought in easyJet and Ithaca Energy is followed by more changes.