U.S. online sales this holiday shopping season are up 15 percent compared to last year, after what might have been the busiest week of the season.

Shoppers have spent $30.9 billion online from Nov. 1 through Dec. 16, up from $26.9 billion last year, said the Reston, Va., company comScore, which tracks Web use.

Online sales surpassed $1 billion on four days last week. Total sales for the week climbed 15 percent to $6.31 billion.

But “Cyber Monday,” the Monday after Thanksgiving, is still the largest online shopping day ever, according to comScore. Sales for that day rose 22 percent from last year to $1.25 billion. Cyber Monday sales topped $1 billion for the first time last year.

Discounting and promotions such as free shipping also have boosted shopping this year.