When studios say that every day is like Saturday between Christmas and New Year’s, they aren’t exaggerating — especially this year.The potential windfall of holiday period weekdays was evident on Monday, when 20th Century Fox’s “Avatar” grossed $19.4 million from 3,456 theaters, bringing the domestic cume to $232.1 million in 11 days. The 3D sci-fi fantasy is fast headed for the $300 million mark. Overall, ticket sales were up 43% over the same Monday a year ago, when Fox’s “Marley and Me” led with $7.5 million, followed by “Bedtime Stories” with $6.4 million. Total grosses that day topped out at $42 million; this year, they reached $60.7 million. Fox’s “Alvin and the Chipmunks: The Squeakquel” placed No. 2, grossing $12.1 million from 3,700 runs for a four-day domestic cume of $87.7 million. Warner Bros.’ Robert Downey Jr.-Jude Law topliner “Sherlock Holmes” came in No. 3, grossing $11.2 million from 3,626 theaters for a four-day cume of $73.6 million. Nancy Meyers’ romantic comedy “It’s Complicated,” from Universal, grossed $4.3 million from 2,887 for a four-day total of $26.4 million. Film stars Meryl Streep, Alec Baldwin and Steve Martin. Disney holdover “The Princess and the Frog” placed No. 5 on Monday, grossing $3.1 million from 3,475 runs for a domestic cume of $66.8 million.