SAN FRANCISCO  On Monday, somewhere in New York City, 420 people will gather for High Times magazine’s annual beauty pageant, a secretly located and sold-out event that its sponsor says will “turn the Big Apple into the Baked Apple and help us usher in a new era of marijuana freedom in America.”

They will not be the only ones partaking: April 20 has long been an unofficial day of celebration for marijuana fans, an occasion for campus smoke-outs, concerts and cannabis festivals. But some advocates of legal marijuana say this year’s “high holiday” carries extra significance as they sense increasing momentum toward acceptance of the drug, either as medicine or entertainment.

“It is the biggest moment yet,” said Ethan Nadelmann, the founder and executive director of the Drug Policy Alliance in Washington, who cited several national polls showing growing support for legalization. “There’s a sense that the notion of legalizing marijuana is starting to cross the fringes into mainstream debate.”

For Mr. Nadelmann and others like him, the signs of change are everywhere, from the nation’s statehouses  where more than a dozen legislatures have taken up measures to allow some medical use of marijuana or some easing of penalties for recreational use  to its swimming pools, where an admission of marijuana use by the Olympic gold medalist Michael Phelps was largely forgiven with a shrug.