Since 1990, when Scotland became one of five countries in Europe to embrace this idea during Glasgow's stint as European City of Culture, it has happened more frequently around the UK, with Wales and Northern Ireland signing up in 1991, and England following three years later.
Thousands of charging museums, stately homes, and other sites of historic and cultural interest now take part every year in European Heritage Days - a Council of Europe initiative designed to reach people from all backgrounds regardless of culture, faith or ability to pay.
In Britain, the annual promotion has a different name in each area. Northern Ireland still favours the official European Heritage Days title, but Wales has recently rebranded it Open Doors.
Scotland uses the term Doors Open Days, while in England the programme - jointly coordinated by English Heritage and the Civic Trust - is known as plain old Heritage Open Days. London, meanwhile, has a separate initiative, Open House, focusing on the evolution of the capital's built environment.
While it is up to the managers of individual properties to decide if they are prepared to waive entry fees, or open areas not normally accessible to the public, the number of participating venues is rising steadily every year. More than 5,000 will take part this month - 65 per cent more than five years ago - including nearly all the 300-plus properties owned by the National Trust.
Among the newcomers are a recently restored 18th-century Gothic banqueting pavilion precariously perched above a steep gorge near Harrogate, and Ayr United Football Club, which is offering a tour of its 120-year-old stadium before its impending move to a new purpose-built one.
The programme also features numerous events alongside the free-openings - ranging from Buggyfit, a pram-pushing tour of landmarks around Milton Keynes for new parents, to guided heritage walks around oft-maligned towns such as Slough and Watford.
"What interests people is that by heritage we're not only talking about the past," enthuses Katja Condy, Civic Trust Heritage Open Days manager. "We open everything from castles and museums to CCTV control rooms, carparks, and ventilation shafts."
But there is evidence that the word is spreading beyond heritage aficionados and culture vultures. According to analysis of last year's visitor surveys in England, of the 900,000-plus visitors recorded over the free period one in three were from C2DE backgrounds, while the same proportion had not visited a heritage site in the previous 12 months.
The open days also appear to be motivating people to take an interest in heritage on their doorsteps. Nearly three-quarters of respondents said the journey to an attraction had taken them 20 minutes or less.
Visits from ethnic minority communities have risen fivefold in the past two years. Unlike cheap flights (which research shows are used by frequent flyers, rather than the less well-off), heritage days appear to be benefiting their target audience.
Free for all?
But closer scrutiny of the ever-lengthier programmes issued by various regional bodies reveals some discrepancies . While the National Trust has free entry to nearly all its properties for one day, the scheme's main architect English Heritage only waives about a third of entry fees to its properties.
And Condy at the Civic Trust says she is having limited success courting owners of stately homes outside the National Trust. "The Historic Houses Association is working hard to get its members on board, but it's a struggle. There's a fear, particularly if they already run a tight ship, that if they lose out on a day's income they'll run into problems."
The national museums in the capital have also been slow to embrace London's Open House and extend free opening from core displays to paid-for temporary shows. Those that don't participate point towards other free events they run. A Victoria and Albert Museum spokeswoman cites its programme of family workshops.
"Last year we had Kylie: the Exhibition, which was free, at the same time as a major exhibition on Surrealism and Design and a free historical show about little-known designer James 'Athenian' Stuart."
A British Museum spokeswoman mentions the free behind-the-scenes tours it runs over Open House weekend, but says it has no plans to waive paid-for exhibition fees.
Despite this one in five heritage day events now involve museums and galleries. This year's participants range from Ambleside's Beatrix Potter Gallery, which is hosting an exhibition on recent Hollywood film Miss Potter, to the Baltic in Gateshead, which will run daily guided tours focusing on its past life as a flour mill.
Long-term rewards
In September 2006 English Heritage opened all its properties free as part of that year's nationwide History Matters campaign but since then it has allowed its site managers to make their own decisions about taking part.
Catherine Bloodworth, acting head of the English Heritage outreach team, says 150,000 people have benefited from the scheme, which has extended open days to a number of areas with high levels of social deprivation, including Easington, County Durham, and Nottingham's St Ann's ward.
Achievements have included persuading various faith communities to take part, with Buddhist and Hindu temples, gurdwaras, mosques, synagogues, and a Jain centre all now opening publicly.
"We concentrate on building up the scheme in new priority areas, encouraging sites that aren't normally open to open up, and increasing participation from under-represented groups," she says.
Participating venues do face costs associated with losing ticket income, and taking on extra staff to cope with numbers, but there are rewards. Marian Gwyn, learning and communications manager of the National Trust-owned Penrhyn Castle, near Bangor, which saw its typical September visitor target of 900 quadruple on last year's open day, insists it's a huge boon.
"We get people bringing entire families. It's not unusual to have four generations in one go, and you often find there's a family connection with the property," she says.
Last year's genealogical theme, under the banner Your History Matters, attracted visitors to the castle whose memories have enhanced Gwyn's own knowledge of the property. One family was related to a former messenger boy and baker, Frank Matthews, employed on the estate at the turn of the 20th century.
Another visitor, Nora Felton, travelled from Liverpool to recount her days living in the castle's garden house as a second world war evacuee. "We didn't know the bothy had been used during the war. The last Lord Penrhyn died in 1949 -before the trust took over - leaving his records to Bangor University, and it hasn't catalogued them yet," says Gwyn.
James Morrison is a freelance journalist
UK properties taking part in open days for the first time include:
Shaw House
Highlights: Berkshire's largest Elizabethan manor house, fresh from a £6m restoration. Visit a first-floor room where a bullet narrowly missed Charles I's head
HarperCollins archive
Highlights: tours of publisher's East Dunbartonshire archive to mark the 500th anniversary of printing in Scotland. Exhibits will include Agatha Christie and JRR Tolkien's letters
Spitalfields Charnel House
Highlights: explore a recently rediscovered 14th-century charnel house in London's historic Spitalfields district
Y Plas
Highlights: inaugural open day of the former home of the Marquess
of Londonderry in Machynlleth
Lister Lane Cemetery
Highlights: a disused Victorian cemetery in Halifax where leading Chartists and figures from the Industrial Revolution are buried
Willow Tea Rooms
Highlights: the manager of Glasgow's Willow Tea Rooms, designed by Charles Rennie Macintosh, will show visitors around the rooms in full Edwardian dress
2008 opening dates
Heritage Open Days in England, 11-14 September, www.heritageopendays.org.uk
Doors Open Days in Scotland held over four weekends 6-7, 13-14, 21 22 and 27-28 September, www.doorsopendays.org.uk/opendays
Open Doors in Wales, 6-7, 13-14 and 20-21 September, www.civictrustwales.org/ehd
European Heritage Days in Northern Ireland, 6-7 September, www.ehsni.gov.uk
London Open House, 20-21 September, www.londonopenhouse.org