It might be the world's most widely planted grape variety, but until now the humble grenache grape has been forced to play second fiddle to better-known varieties such as cabernet sauvignon and pinot noir. The organizers of International Grenache Day call the grape "the unsung hero of the wine world," with "robust, sturdy vines that produce grapes of great character."

They've resolved that grenache must no longer be left in the shade, so a day of celebrations devoted to its charms has become an annual event.

The drive to promote grenache was first launched at an international symposium held in the southern Rhône in 2010. The 250 wine producers and industry professionals who attended said they believed that grenache-based wines were "sometimes misunderstood, underestimated and/or undervalued. We aim to right that wrong."

Grenache (garnacha) is grown in vineyards all over the world, from France to Australia. Its French strongholds are the southern Rhône and Roussillon. Most famously, the grape is included in the wines of Châteauneuf-du-Pape and the Spanish region of Priorat. British wine critic Jancis Robinson told the grenache symposium she had always wondered why tempranillo seemed to attract more reverence than garnacha. "I suspect it was simply that garnacha was so widely planted and was therefore regarded almost as a weed, whereas tempranillo seemed more exotic and therefore noble."

Robinson's good-value grenache picks are old-vine garnacha wines grown in Spain’s less famous regions, such as Calatayud and Campo de Borja. In her view, "These wines have wonderfully attractive juicy, spicy fruit, and can be charming even in extreme youth."

Grenache lovers are being invited to mark the day by tweeting (#GrenacheDay), blogging about the grape, wearing an appropriate T-shirt, or even – the old-fashioned way – by simply opening a bottle of appropriate wine.

In California, grenache events are being held in Paso Robles, Sacramento, Santa Cruz and Santa Rosa, while the state's "Rhone Rangers" got in ahead of time with a two-hour live Twitter tasting on September 20. The Rhone Rangers represent 150 wineries which make wines from one or more of the 22 grape varieties used in the Côtes du Rhône – including grenache.

“Grenache, in all its forms, is one of the most exciting grapes in America,” said Rangers president Josh Bendick, co-winemaker at Holly's Hill Vineyards. “New plantings of grenache noir have topped 1,200 acres in California alone in the last decade, and grenache blanc is the fastest-growing white Rhone variety in America.” Holly's Hill produces two grenache varietal wines, El Dorado Grenache Noir and El Dorado Grenache Rosé.

Across the Pacific, the McLaren Vale wine region – the center of grenache production in Australia – is holding an International Grenache Day Tweet Up. Among the wineries joining the celebrations is d'Arenberg, whose grenache portfolio includes The Custodian, The Derelict Vineyard and The Blewitt Springs.