Many online and live-service games utilize some form of a daily reset for various purposes. In the case of Splatoon 3, it is to refresh the inventory for the various shops. Because rather than make everything available all at once, Nintendo, like many others, believes they can drive player engagement up with FOMO (Fear of Missing Out). Here's when that daily reset occurs.
When is Daily Reset for Splatoon 3?
The Splatoon 3 Daily Reset occurs at 8pm ET This means that players can peruse the new offerings for headgear, clothing, shoes, and the general store. This also means that Murch will have new deliveries coming in. Make sure to check with him so that you don't miss your order!
It's not an ideal system as we would rather just have everything available right out of the gate. There doesn't appear to be a way to view everything in the store, so it's quite literally a random roll of the dice for the offerings. It is even more frustrating that sometimes the items will repeat from one day to the next. Nintendo has a great game on their hands and handling gear, especially cosmetics, which some players think feels predatory. Regardless of anyone's feelings, at least you know when to check for new gear.
For more Splatoon 3 goodies, check out How to view the entire Catalog in Splatoon 3 on Pro Game Guides.