On the eve of "Cyber Monday," online retailers are reporting a strong start to the holiday shopping season.

Research firm comScore reported on Sunday that e-commerce spending jumped 26 percent on Black Friday, the day after Thanksgiving, compared with the same day a year ago. ComScore reports $816 million in online sales for the day.

Thanksgiving is also a big day for online sales, and comScore reports an 18 percent increase this year compared with a year ago, with $479 million in sales.

Online sales also have been strong throughout November. ComScore reports a 15 percent increase in online sales through Saturday, with a total $12.74 billion.

Next up is Cyber Monday, when many online retailers run promotions for the first business day of the week following Thanksgiving.