Families are set to spend more than £100 per household this summer celebrating the Diamond Jubilee and the Olympics and Paralympics, according to research.
A quarter of those surveyed plan to mark both the Queen's 60-year reign and the sporting occasions while half will toast either one event or the other, said the Money Advice Service.
The average spend on jubilee events will be £112 per household and those celebrating the Olympics and Paralympics will spend around £208, it estimated.
Four in 10 are likely to spend more money than usual this summer, while 15% said they will push the boat out and spend "a lot more".
Six in 10 are unclear about exactly how much their celebrations will cost and 17% have no idea.
The Money Advice Service, an independent body set up by the Government, has launched its Big Summer campaign to help people budget for the cost of this summer's events.
Sarah Smith, behaviour expert at the service, said: "2012 looks set to be an unforgettable summer, from the jubilee and the London 2012, to everything in between from day trips and holidays to exciting sporting events.
"It's troubling to hear so many people are unaware of their summer spending. Costs can rack up, so it's important to get a grip and avoid unexpected expense."
Around £3 billion will be spent by 23 million households this summer as people either go to watch an event live, celebrate it at home with family and friends or attend a street party, the service said.
The study involved 2,101 people from across the UK. The Big Summer planner, which has an online calculator, is at moneyadviceservice.org.uk/bigsummer.