This year’s Cyber Monday became the largest online shopping day in U.S. history.

According to Adobe Insights, online transactions that day reached a record $6.59 billion. That’s a 16.8 percent increase from 2016’s Cyber Monday sales.

And it looks like people are increasingly using their mobile devices to do their shopping. Mobile sales reached $2 billion in 2017.

“This year, mobile shopping was dominant both in the morning and afternoon, and desktop only staged a comeback in the evening when people were home,” Taylor Schreiner of Adobe Insights told CNBC.

This is due to two factors: first, retailers have worked really hard to optimize their sites for mobile viewing, and second, people are figuring out how to use their mobile devices to do their shopping.

In the face of increasing traffic by people using mobile phones and tablets, it seems that retailers have seen the light and decided that if they want to compete with behemoths like Amazon and Walmart, they should create mobile-optimized websites.

The huge amount of money spent on Cyber Monday by people using their devices—almost a third of the day’s sales—shows that people are becoming increasingly comfortable with using tablets and phones to do shopping, in addition to other daily activities.

It looks like the holiday shopping season is going to be good to retailers, at least the online ones. From November 1 through November 27, $50 billion was spent online, and experts say spending is on track to reach $100 billion by the end of the year.

Cyber Monday wasn’t the only big online shopping day, either. Black Friday’s online sales also set a record at $5.30 billion, an increase of 17 percent over last year’s online sales.

Who were the big winners on Cyber Monday? Amazon, naturally; sales on that site accounted for 45 to 50 percent of all sales by volume. eBay had the second largest volume of sales, and Walmart came in at Number 3.

Brick-and-mortar retailers also did well over the weekend. Macy’s, Kohl’s, and JCPenney also reported strong or record-setting sales over the Thanksgiving weekend.