The sharp sell-off in U.S. stocks late on Wednesday jolted those investors who had been sitting back hoping to enjoy the 'everything rally' until the end of the year.

It could take until U.S. PCE inflation data on Friday to say definitively whether this was the cold reality of January arriving early, or just a metaphorical late-Christmas morning squabble soon solved by dinner, but Thursday's open will still be interesting to watch.

S&P and Nasdaq futures are up around 0.5% at the time of writing, so the initial signs are that Wednesday's mid-afternoon nosedive was just a blip.

All three major U.S. stock indexes ended Wednesday 1.3% to 1.5% below Tuesday's close, having been pootling along quite happily until the sudden turn lower. Asian and European stocks sold off in sympathy on Thursday.

A fall was not hugely surprising given the scale of the recent rally, but why then and why so sharp has left many market watchers scratching their heads.

"The strategists text book would suggest that we point to 'year-end liquidity issues'" said Rabobank European rate strategists in a morning note.

Poor results from FedEx - seen as a bellwether for economic activity - couldn't have helped, but as they were released after market close on Tuesday they aren't much use to explain the timing.

Analysts are also pointing to markets belatedly responding to attacks on vessels in the Red Sea by Iran-backed Houthi militants, causing exporters to scramble to find alternative routes.

Maybe the thing to do is to remember it's nearly Christmas and the S&P 500 is still up 9.5% so far since the start of October, which would be its best quarter in two years.

The juggernaut of market euphoria from last week's dovish shift in the Federal Reserve outlook keeps rolling on in the rates market however.

The benchmark 10-year U.S. Treasury yield was last at 3.8676%, near the previous session's five-month low of 3.8470%.

In Europe, Italy's 10-year yield is at 16-month lows, given another nudge by European Union finance ministers agreeing the latest reform of the bloc's two-decade-old fiscal rules.

The day ahead is pretty thin on the data front, with final third quarter GDP data and weekly jobless claims. The last big piece of data before Christmas is Friday's U.S. core PCE price index reading - the Fed's preferred measure of underlying inflation, where another slowdown is expected.

Key developments that should provide more direction to U.S. markets later on Thursday:

- U.S. weekly jobless claims

- Third quarter U.S. final GDP

- Canadian retail sales

- ECB chief economist Philip Lane speaks on a panel

(Reporting by Alun John; Editing by Emelia Sithole-Matarise)