Every spring, a version of the same message arrives: sell in May, sit out the summer, come back when the nights draw in. It sounds like an easy win — skip the dull months, keep the good ones. Markets rarely hand out free money on a calendar date, though, and the UK version comes with frictions the slogan cheerfully ignores.

So what does the pattern actually claim, what do the long-run numbers suggest, and is there a way to use it without turning a portfolio into a guessing game?

What the rule says, and why anyone believed it

The rhyme — "sell in May and go away, come back on St Leger's Day" — has been part of City folklore for a very long time. The reasoning was practical: London's trading floors emptied in summer as merchants and the gentry left town, liquidity thinned, and activity picked up again in the autumn. Whether that was ever the real cause, the observation stuck. Later work on long-run index data found something similar across several developed markets: the November-to-April stretch has tended to deliver stronger returns than May to October. The pattern is often called the Halloween effect.

Two things are worth noting. First, "tended to" is doing a lot of work here — that is a statement about averages across many decades, not something that repeats every year. Second, the pattern describes a difference in returns, not a guarantee that summer is negative. Plenty of summers have been very good indeed.

The mild version and the drastic version

There are two ways to run it. The mild version trims equity exposure in May, parks the difference in cash or short-dated bonds, and buys back in the autumn. The drastic version sells everything for six months and tries to time the re-entry. The first can be defended as a tilt. The second turns a long-term investor into a market timer, which is a much harder job than it looks.

What the UK record actually shows

UK indices do show a seasonal tilt, but it is modest, unstable, and sensitive to how you measure it.

• The dates are arbitrary. Shift the window by a few weeks and the results change materially. A pattern that only works with the "correct" start and end dates, chosen after the fact, is not much of a strategy.
• A handful of years do the work. Long-run averages are often driven by one or two dramatic summers or autumns. Take those out and the gap narrows sharply.
• The gap has narrowed over time. Electronic trading, global fund flows and round-the-clock news mean the old summer lull matters far less than it did when the rhyme was coined.
• Dividends complicate the picture. Total returns for larger UK companies lean heavily on dividends, and those arrive throughout the year. A price-only chart tells a different story from the return an investor actually receives.

There is also a UK-specific point. The FTSE 100 is a global index wearing a British label: miners, oil majors, banks and pharmaceutical companies whose earnings come from China, the United States and continental Europe. Its summer behaviour has more to do with industrial demand and interest-rate expectations than with whether London is on holiday.

The costs of stepping aside

Every switch costs money, and those costs are certain while the benefit is not. On many UK share purchases you pay stamp duty reserve tax at 0.5%, plus dealing commission and the dealing spread. In a taxable account, selling can also crystallise a gain that may be subject to capital gains tax. Inside an ISA or a pension the tax friction mostly disappears, but the dealing costs do not. Do this twice a year for a decade and you have paid real money for an unproven edge.

Then there is the risk you did not plan for: being out of the market on the days that matter. Annual equity returns are lumpy, with a small number of strong days accounting for a large share of the total. A sharp rebound in late June or a rally triggered by a surprise on interest rates can arrive precisely when you are sitting in cash — and now you face the choice of buying back higher or waiting and hoping.

Behaviour is the bigger risk

Seasonal rules are easy to describe and hard to follow. They ask you to act twice a year, usually against whatever the market has just done, and they leave plenty of room for a quiet amendment: "just this once, I'll wait until the Budget." Do that often enough and the plan dissolves into discretionary trading with worse record-keeping.

A seasonal rule you abandon in the second year is worse than no rule at all.

A practical way to handle seasonality

If you want to test the idea, do it in a way that cannot do much damage:

1. Use a tilt, not a switch. Move a small slice of the equity part of your portfolio — not all of it. If the pattern fails, the damage is a rounding error rather than a decade of return.
2. Write the rules down before May. Exact dates, exact size, and the conditions for buying back. "I'll know it when I see it" is not a rule.
3. Count the costs first. Add up dealing fees, stamp duty, spreads and any tax. If those exceed the historical gap you are chasing, stop there.
4. Prefer cash flow to selling. Redirect new monthly contributions into cash over the summer instead of selling existing holdings. You get the same tilt without triggering a disposal.
5. Keep a benchmark. Compare the seasonal version of your portfolio against what you would have had by doing nothing at all, and be honest about the result.
6. Give it years, not one summer. A single year tells you nothing. If you are not willing to run the experiment for a decade, it is probably not worth running.

What matters far more than the month

For most UK investors saving regularly into an ISA or a pension, the variables that genuinely move the outcome are contribution rate, asset allocation, total costs, and behaviour under pressure. Holding cash for six months a year is a drag on returns whenever markets rise, and it has to be paid for out of a seasonal effect that may not show up at all.

Interest rates, inflation, sterling, valuation and dividend cover will shape your returns far more than the month on the calendar. Worth remembering when a rhyme starts to feel like a plan. This is general information rather than personal advice; if a decision involves a large sum or a taxable account, a regulated adviser can look at your circumstances properly.

The sensible default for UK investors

Sell in May is not a broken clock, but it is not a system either. The pattern appears in long-run averages, it is weaker and less reliable than the slogan implies, and the costs of acting on it can swallow whatever edge remains. For most people building a portfolio over decades, the answer is to keep contributing, stay diversified, and treat seasonality as a

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