Will Gau holds his glass of ruby chinon to the light and tastes. "It's strict, a little dusty, a little jagged," says the wine connoisseur. But his disappointment may not be down to bad selection or the bottle being corked but to an altogether more cosmic force: the moon.

The idea that the taste of wine changes with the lunar calendar is gaining credibility among the UK's major retailers, who believe the day, and even hour, on which wine is drunk alters its taste. Tesco and its rival Marks & 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 suggests they will show at their best.

Marks & Spencer has 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 calendar has been published for the last 47 years by a gardening great-grandmother called Maria Thun, who lives in rural Germany. She categorises 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 today to 10pm tomorrow. Those who believe in the theory admit it has overtones of "druids dancing in the moonlight" but believe the effect can be explained by considering 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. So many remain confused about screw caps, for example. We don't want to make it more complicated."

Jo Ahearne, winemaker for Marks & Spencer, became convinced of the theory when she sampled more than 140 wines over two days. "Before the tasting, I was really unconvinced, but the difference between the days was so obvious I was completely blown away." The Guardian tested the theory this week and tasted the same wines on Tuesday evening, a leaf day, then again on Thursday evening, a fruit day. Five out of seven bottles showed a marked improvement.

"I was sceptical but I think the evidence was overwhelming," said David Motion, the London wine merchant who hosted the tasting. "I live in the city and don't think much about nature but it is clear it has an influence. The cosmos is forcing its way in."

In other quarters, doubts remain. Waitrose's wine department has investigated the idea and cannot see a correlation. Many scientists have little time for biodynamic wine, pointing out that the movement's guru, Rudolf Steiner, claimed to have conceived the concept after consulting telepathically with spirits beyond the realm of the material world. Among his other works are claims that the human race is as old as the Earth and descended from creatures with jelly-like bodies, and a belief that men's passions seep into the Earth's interior, where they trigger earthquakes and volcanoes.

Biodynamic winemaking is well established in France, Germany and the USA, and it mostly means avoiding chemical fertilisers and pesticides and encouraging biodiversity.

It also involves spraying the vines with preparations which sound more like witches' potions than agricultural aids. One involves fermenting cow manure in a cow horn, buried underground over winter. In another, oak bark is fermented in the skull of a domestic animal.

The wine week

The biodynamic drinking forecast for the next seven days:

With most wines showing best on fruit and flower days, drinkers face a tricky weekend. From 8am today until 10pm tomorrow is a root day. Flower day starts at 11pm tomorrow. In the next week, the best drinking time looks to be from 8pm on Wednesday until 11am on Thursday and from 6pm on Friday and throughout Saturday - both fruit periods. Lunchtime on Wednesday looks the worst time for wine.