When temperatures rise, sales slide, web analytics firm SumAll says.

When consumers start reaching for sun block and ice cream rather than their keyboards, that shows up on e-retailers’ bottom lines. An analysis of summertime online shopping habits by web analytics firm SumAll shows gross sales for online merchants in July decline 30% from December highs, and average order value falls nearly 5% below the annual average in August.

That’s based on four years of summertime spending data collected from 3,000 merchants that use SumAll’s web analytics program. SumAll says the data account for nearly $500 million in online sales.

The data reveal consumers spend less per order in the summer months. SumAll calculates average order value on an annual basis at $86.07, but in July that falls 2.8% to $83.67 and in August 4.8% to $81.87. SumAll recommends that if e-retailers are going to take a summer vacation, they do so the week of July 23, the week when sales are the slowest. It advises that e-retailers be back and ready to sell by the week of Aug. 27, when summer sales peak as back-to-school shopping comes into play. Sales that week are 19% higher than during the week of July 23.

Wednesday is the top sales day of the week during the June to August timeframe, with e-retailers collecting 16.1% of weekly revenue on that day. Mondays generate 15.9% of weekly sales, followed by Thursdays, with 15.7%. Saturdays and Sundays—prime beach days—contribute the least to weekly sales; the two days account for 22.3% of weekly sales. SumAll does not say how these sales patterns vary from the rest of the year.

SumAll says consumers spend the most per item online on summer Mondays ($41.13) and the least on summer Fridays ($34.74). The company recommends adding sales incentives, such as free shipping for orders of $100 or more, on Mondays to push consumers to make higher-value orders.