Now that the sun has set on Black Friday, Cyber Monday is on the immediate horizon.

Black Friday used to be the undisputed king of shopping days, but according to the National Retail Federation, the day after Thanksgiving is only the start of a consumer buying bonanza that lasts until Christmas. That weeks-long stretch brings in between 30 to 40 percent of all annual retail sales, the NRF states, and this year holiday retail sales are expected to surge between 3-4 percent, up from $655.8 billion last year.

According to retail expert Jan Kniffen, the boost to retail sales will be even more pronounced, thanks to online buying. "Online is going to grow 20 percent this year in the holiday season," Kniffen predicted to CNBC's "On the Money" in an interview.

"But in the battle for your holiday retail dollar, when will online buying overtake in-store sales?" he asked. "That has to happen, because we're gradually going to be 50 percent online (sales) in this country," Kniffen added.

Indeed, anecdotal evidence suggested mobile sales set a new record on Friday, data from Adobe Insights showed.

Part of the reason could be disenchantment with the spectacle of Black Friday, which has begun encroaching on Thanksgiving proper.

"Even before stores were open all kinds of crazy hours and round-the-clock hours over the Thanksgiving weekend, Black Friday was always your highest traffic day, but it was frequently not your highest sales day," Kniffen told CNBC.

He said families shopped all day, "…so there were lots of people going through your store. It was the original beginning, I think of 'showrooming'," Kniffen said. "They all came in and looked at the stuff but they came back and bought it later; now that happens online."

While "brick and mortar [retail] is a much bigger business…we have reached the tipping point" he said, as in-store purchases defy steep discounts and foot traffic falters. Kniffen noted that in the fourth quarter of last year during the holiday shopping season, "traffic was down 8 or 9 percent in the enclosed malls and we saw growth online of almost 20 percent."

It's an unsustainable dynamic, he explained. "Because it's really hard to run your expense structure when you're not getting growth in the stores."