Today is a big day for marijuana in the United States. April 20, in case you hadn't heard, is pot-smokers' national "holiday." The number 420 has been connected to pot smoking since the 1970s for reasons that have always been a bit, well, hazy. But that's never stopped marijuana aficionados from adopting the date as their own, and this year they have good reason to think they're on the winning side of America's drug war.

For the people who have flocked to Denver for Sunday's 420 Rally, this weekend is nothing less than a celebration. Since Colorado voters legalized marijuana for recreational use in 2012, Colorado has become America's Amsterdam, with the first shops opening this Jan. 1. The law is increasingly making the 420 Rally a Mardi Gras of the Rocky Mountains, packed with four different music festivals and the Cannabis Cup – a gigantic emporium for the world's best marijuana paraphernalia that sold out its 37,000 tickets.

Yet the potential success of Colorado's great pot experiment is not in the exuberance of those in Denver today. It is in dollars and cents and law and order.

So far, Colorado has taken significant strides toward showing that it can create a taxable industry that threatens neither public health nor public safety. Even US Attorney General Eric Holder told the Huffington Post this month that he was "cautiously optimistic."

Washington State is next. Voters there, too, legalized recreational use of marijuana in 2012, and the state has issued its first pot-selling business license, though no shops are open yet. If these two states can succeed, the trend could only spread.

New Jersey state Sen. Nicholas Scutari, a Democrat, gave a hint as to why when he proposed legalizing marijuana for recreational use in his state. "Scutari cited tax revenue netted in Colorado, which legalized weed this year, and estimated Jersey’s haul at $100 million a year," wrote the New York Daily News.

A New York Times/CBS News poll found that 51 percent of Americans support legalizing marijuana. That is up from 27 percent in 1979. If that number continues its upward trend, as expected, the tax money that legal marijuana promises politicians will look more inviting. For its part, Marijuana Business Daily forecasts that industry sales nationwide will hit $8 billion in 2018.

The potential expansion of legal marijuana has already largely played out with medical marijuana. Twenty-one states currently allow marijuana for medical use. Twelve more will consider it this fall, meaning more than half of states will likely have some form of legal marijuana by the end of the year.

Only two states, however, are likely to consider legalizing marijuana for recreational use this fall. The question is already on the Alaska ballot, and advocates in Oregon have vowed to gather enough signatures by July to put it on the ballot there. The Oregon effort is led by Paul Stanford, one of the nation's leading advocates for legalizing marijuana. Meanwhile, a February poll by Public Policy Polling found 55 percent support for legal recreational marijuana in Alaska.

Advocates of legal marijuana in other states, including California, Massachusetts, and Nevada, appear to be targeting 2016.

The rollout of legal marijuana in Colorado has not been without problems. In March, a middle school student brought edible marijuana candies to school and passed them out to students. And Nebraska law enforcement officials complain that marijuana cases are skyrocketing.

The Star-Herald of Scottsbluff, Neb., reports that "Deuel County, which is where eastbound Interstate 76 traffic from Colorado enters Nebraska and merges onto I-80, has about 30 felony marijuana cases attributed to Colorado pot on its court ledger so far this year. There were about 35 cases total last year."

Law enforcement in Utah, Wyoming, and Oklahoma, however, have not noticed similar spikes, according to the report.

Attorney General Holder suggested that how Colorado and Washington State handle their legal markets will be crucial to how far legal marijuana spreads. "I think a lot of states are going to be looking to see what happens in Washington, what happens in Colorado before those decisions are made in substantial parts of the country," he told the Huffington Post.

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Until then, Denver's 420 Rally looks set to keep growing, at least.

The Denver Post writes: "April 20 is now a full-on entertainment holiday in Colorado with promoters and entrepreneurs looking to capture a slice of the massive audiences in the same way they might on Halloween or New Year's Eve...."