U.S. online shoppers shelled out $7.9 billion across all retailers on the first day of Amazon's four-day Prime Day sale, up nearly 10% from last year, per Adobe data.Why it matters: Prime Day is more than just a summer shopping event — it's a pulse check on U.

Prime Day shopping events to entice cautious consumers and play defense against stiffer competition from the likes of Walmart Inc., Target Corp. and TikTok, who have launched their own discount days in response.

Momentum Commerce, which manages online sales for 50 brands, said its Amazon purchases plunged 41% on Tuesday when compared with the start of Prime Day last year.