Twentieth Century Fox’s “Alvin and the Chipmunks: The Squeakquel” opened to $18.7 million on Wednesday from 3,656 theaters, securing the best gross for a title debuting on the Wednesday of Christmas week at the domestic B.O.One Fox insider say the studio didn’t expect “Squeakquel” to outdo Fox’s “Avatar,” which still did boffo busines, grossing $16.4 million from 3,452 runs for a domestic cume of $125.9 million. “It was a great number for ‘Alvin,’ and it exceeded our expectations,” Fox senior VP of domestic distribution Bert Livingston said. Original “Alvin and the Chipmunks” posted an opening day gross of $13.3 million from 3,475 runs when opening Dec. 14, 2007, a Friday. That film cumed $279.2 million domestically. Previous best opening for the Wednesday before Christmas appears to be “Meet the Fockers” at $12.1 million, according to Rentrak. Between “Avatar” and now “Squeakquel,” Fox is ending the year on a high note. “Squeakquel” has the advantage of being the only new family entry at the holiday B.O. James Cameron’s “Avatar” is displaying extraordinary B.O. strength, grossing at least $16 million every day this week. On Wednesday, it was one of the titles that was up from Tuesday at 2% “Avatar” gets competition on Christmas Day from Warner Bros.’ Robert Downey Jr.-Jude Law topliner “Sherlock Holmes.” Theater traffic is expected to slow dramatically today, being Christmas Eve. “Alvin” could be heavily impacted.