Weather forecasters say the sun will shine across much of England and Wales ahead of the spookiest night of the year.
The mercury could rise to 19C (66.2F) in some places - significantly higher than the 12C-13C (53.6F-55.4F) maximum UK average for the month of October.
Grahame Madge, spokesman for the Met Office, said: “It’s going to be very mild actually. In the south, we’re looking at temperatures in London of around 18C (64.4F) possibly, possibly even a degree above that, possibly even 19C in the odd place.
“Obviously it’s going to cool down overnight but still we should have temperatures in the higher single figures.”
He said it will be largely dry throughout and fog will clear, adding: “Quite a lot of sunshine developing for most of England and Wales.”
However, there is not such good news for the north of Scotland, where Mr Madge said forecasters expect rain.
Maximum day temperatures for Halloween in previous years are 20.8C (69.4F) in 2015, 23.6C (74.5C) in 2014, 15C (59F) in 2013 and 13.8C (56.8F) in 2012.
Meanwhile, fog caused disruption at Heathrow Airport due to poor visibility.
A spokesman for the airport said it has had at least 15 rotational cancellations (out and back) and several delays because of the poor weather, adding that these are mainly BA short-haul flights.
He said the fog continues to be “quite intense” and is expected to have an impact on the schedule for much of the day.
“Our teams are working with airlines to support passengers who have been affected get on the next available flights, and we are also assessing the weather over the next few days,” he said.