The three gardens I visited yesterday were open on the NGS (National Garden Scheme) -- a remarkable organization that promotes openings of private and public gardens for charity, usually one to several days a year. The selection process guarantees that the gardens are all visit-worthy.
They're published in an annual "Yellow Book" with garden descriptions and opening times. People clearly choose times good for their gardens, but they're usually on Saturdays, Sundays, or Bank Holiday Mondays.
Looking at the maps of the different areas of the UK, and the numbers and densities of these gardens open on the NGS scheme is impressive -- it's a remarkable initiative, and raises a great deal of money for specific charities.
The first I visited was Gardener's Cottage, the private garden of the Head Gardener for West Dean Gardens and his wife, also a professional gardener. It was breathtaking, really, in how compact and spectacular it was, and almost impossible to reflect in photos. It was intensively planted, with just a small circle of grass, part given over to a wildflower meadow.
The garden visitors were fascinating, too -- they were obviously all keen gardeners, as it was threatening to rain, and their talk was of various aspects of the garden.