Online holiday spending climbed 15% and surpassed $24 billion during the first 39 days of the holiday-shopping season, new data from comScore shows.

The latest growth in online spending at retailers like Amazon.com (AMZN) signals Americans continue to grow comfortable with e-commerce.

According to comScore (SCOR), online holiday spending grew by 15% year-over-year to $26.4 billion during the first 39 days of the November – December 2011 holiday season

Likewise, retail e-commerce spending jumped 15% last week to $5.9 billion and three days surpassed the $1 billion mark.

By comparison, Cyber Monday sales this year soared 22% to a single-day record of $1.25 billion, comScore said.

"These highlights represent another very positive sign for the holiday shopping season, as the week following ‘Cyber Week’ often experiences relative softness in spending momentum due to retailers pulling back on their promotional activity,” comScore Chairman Gian Fulgoni said in a statement. “All signs are now pointing to a strong finish to the season.”

Despite the news, shares of retailers mostly retreated on Monday as U.S. stocks suffered a selloff amid renewed concerns about Europe’s sovereign debt crisis. Shares of the S&P retail ETF declined 1.62%. Discount retailers Wal-Mart (WMT) and Target (TGT) saw more modest losses.  

The news comes on Green Monday, which tends to be the heaviest or one of the heaviest online spending days of the season. Over the past six seasons, Green Monday ended the year as the highest revenue day twice and the second-highest day three times, comScore said.

Fulgoni said Green Monday’s position may slip somewhat over time because Free Shipping Day, which is December 16 this year, gains in importance each season.

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