The taste of wine changes with the lunar calendar, according to experts at leading supermarkets.

Tesco and Marks and Spencer, which sell about a third of all wine drunk in Britain, now invite critics to taste their ranges only at times when the biodynamic calendar suggest they will be at their best.

Marks and Spencer has even gone a step further and is advising customers to avoid disappointment from the best bottles by making sure not to open them on "root" days.

The cosmic calendar has been published for the last 47 years by a gardening great-grandmother called Maria Thun, who lives in rural Germany. She categories days as "fruit", "flower", "leaf" or "root", according to the moon and stars.

Fruit and flower are normally best for tasting and leaf and root worst. This weekend happens to be root, from 8am on Saturday to 10pm on Sunday.

Those who believe in the theory admit it has overtones of "druids dancing in the moonlight" but say the effect can be explained by regarding the wine in a bottle as a living organism which responds to the rhythms of the moon in a similar way to human biology.

The concept is an extension of biodynamic farming in which decisions about when to sow and prune are made according to patterns of lunar and cosmic rhythms. It was developed from a series of lectures given in 1924 by the Austrian philosopher-scientist Rudolf Steiner.

Tesco has used the calendar for more than two years to decide on times for its thrice-yearly critics' tastings but has not shared its belief with customers for fear it will add yet more mystique to wine.

"Our first choice is a fruit day," said Pierpaolo Petrassi, Tesco's senior product development manager. "We seek to avoid root and leaf days. It may be a little step beyond what consumers can comprehend. We have so many other things to educate consumers about. We don't want to make things too complicated.

Jo Ahearne, winemaker for Marks & Spencer, became convinced of the theory when she sampled more than 140 wines over two days.

"The difference between the days was so obvious I was completely blown away," she said.