The mercury is forecast to climb in the southern and eastern parts of England, with large parts Britain basking in sunshine.
There will be some showers in the west and Wales, and cooler weather in Scotland and Northern Ireland.
However, most of the UK will see temperatures in the high 20s.
It means a warm start to the Premier League season - and for those sitting in the terraces at Lords on the third day of the third test match between England and South Africa.
MeteoGroup forecaster Gemma Plumb said: "There is the potential for it to be the hottest day, with the highest temperatures expected to be across East Anglia, south-east England and parts of the East Midlands."
"Temperatures will be between 29C (84.2F) and 31C (87.8F) but I can't rule out it reaching 32C (89.6F)."
Temperatures last peaked on July 25, when it reached 31.4C (88.5F) at the Olympic Park in east London.
Other events taking place this weekend include the V Festival at Hylands Park in Chelmsford and Weston Park in Staffordshire, while Team GB's para-cycling team are due to take part in a 20km time trial at Blenheim Palace in Woodstock, Oxfordshire, as a warm-up for the Paralympic Games.
Sunday is expected to be more unsettled, with some heavy and thundery showers, but temperatures could still reach 30C (86F) in London and 26C (78.8F) in other parts of the country.