And then it peaks in early August

It’s not even July, but if online conversion data from last year is any sign, back-to-school shopping is about to begin in earnest. And that means marketers holding off on campaigns might want to get started sooner rather than later

MediaMath, a programmatic advertising, analyzed back-to-school advertising and spending to understand when the shopping season starts and finishes. The New York-based firm—which works with two-thirds of the Fortune 500—found that online conversions start to climb the week of Independence Day and peak by the beginning of August.

According to the data, spending and conversions in 2016 rose mostly in tandem from July 8 through August 5. However, the fall begins during the “back to school high season” of July 22 through August 25, and then dropping dramatically after Labor Day weekend.

Overall, back-to-school spending is expected to be higher this year than it was last year, according to the National Retail Federation, a trade organization that represents the retail industry. Total spending for K-12 and college could reach $75.8 billion, up from $68 billion in 2016.

While parents of students in kindergarten through high school had the highest conversion periods in late July and early August, they also tended to shop all the way through Labor Day weekend. However, marketers looking to reach college-bound shoppers should run campaigns between early July and mid-August, per MediaMath’s findings. (Last year, shopping peaked by July 29.)

Where the ads appeared mattered along with when they appeared. According to MediaMath’s inventory analysis, ads bought through premium media inventory performed at twice the rate of all media in the back-to-school category. Mobile is also key during the middle of the summer, with site traffic from mobile devices higher from July 10 through August 28 than it is before the summer begins.