"It's high time we made it legitimate," he says of his relationship with partner Brad Altman

George Takei and his soon-to-be-husband Brad Altman are gearing up for their Sunday evening wedding in downtown Los Angeles.

“We’ve been living in sin for the past 21 years,” Takei, 71, tells PEOPLE. “It’s high time we made it legitimate.”

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The wedding party includes two of Takei’s former Star Trek castmates – best man Walter Koenig, who played Chekov on the series, and Nichelle Nichols, who played Uhura. (Takei played Mr. Sulu.)

With T-minus a day until the nuptials, Takei summed up his mood as: “Nervous-ville.”

His partner, Altman, 54, is spending what’s left of the weekend polishing up his vows, which he’ll recite during the couple’s Buddhist wedding ceremony. “This is going to be the most exciting day of my life,” he says. “I’m going to be married. I m the luckiest guy in the world.”

In May, Takei announced his plans to wed after California’s Supreme Court allowed gay marriage under the state’s constitution.