NEW YORK — U.S. online sales in the holiday season to date are up 15 percent to $24.6 billion, according to comScore.

The research firm said Sunday sales on six individual days during the first 39 days of the November-to-December shopping season have exceeded $1 billion, led by Cyber Monday, the Monday after Thanksgiving, when sales hit $1.25 billion.

Sales in the most recent week ended Dec. 9 rose 15 percent to $5.9 billion, the Reston, Va., company said.

“As we enter what will be the heaviest week of the season for online retailers, beginning with ‘Green Monday’ today, all signs are now pointing to a strong finish to the season,” comScore Chairman Gian Fulgoni said.

Over the past six years, “Green Monday” — a term coined by eBay Inc. and traditionally the second Monday of December — has consistently ranked among the top spending days of the season, although it is no longer the top online spending day.