Summer Shells are only found on beaches during the summer months of June, July, and August in the Northern Hemisphere, and December, January, and February in the Southern Hemisphere. Their primary application is as a crafting material for eight seasonal DIY recipes that can only be obtained during the summer months. You can also wear ACNH summer clothes in the hot time, such as summer dresses and T-shirts. To find and obtain Summer Shell crafting materials in Animal Crossing: New Horizons on your island, you must be playing during the Summer season, which occurs in June, July, and August in the Northern Hemisphere and December, January, and February in the Southern Hemisphere. They take the place of the seasonal Young Spring Bamboo that was found in the Spring months before. During this limited time period in the Summer months, you can find Summer Shells randomly spawned on your island's beaches and along the shoreline. They have a distinct blue color appearance in contrast to the regular shells that can be found all year, so they shouldn't be difficult to distinguish from the others.

How to Obtain Summer Shells in a Timely Manner
To find the most Summer Shells as quickly and efficiently as possible, we recommend checking your island beaches throughout the day and approximately once every hour to maximize your chances of finding them. According to our observations, it takes approximately an hour for your beach to fully respawn with new shells. You can also time travel throughout the day to expedite this process.

Keep in mind to pick up regular non-summer shells as well while you're at it to make room for new potential Summer Shells to appear, as you'll need the other shells to craft some of the Summer items. Not to mention that there are a variety of other Shell Set ACNH items that can be created with them.

Summer Shells Sell for a Good Price at Nook's Cranny
Summer Shells can be sold to Timmy and Tommy at Nook's Cranny for 600 Bells each if you do not wish to use them in crafting. You only need about 30 Summer Shells to complete the entire set, and you have the entire three months of Summer to do so, so don't be afraid to put them up for sale.

There are nine different Summer Shell-themed items in Animal Crossing: New Horizons that can only be made with Summer Shell seasonal materials. These items include furniture, tools, accessories, and more. You can obtain these DIY Crafting Recipes by shooting down flying Balloon Presents during Summer Season, which occurs in June, July, and August in the Northern Hemisphere and December, January, and February in the Southern Hemisphere. For veteran players, a new recipe called the Summer-Shell Rug was added in November 2020's Version 1.6 Update, which you may not have had access to by 2021.

DIY Recipes for Summer Shells - How to Get Them
There are two ways to find and obtain new Summer Shell DIY Crafting Recipes in Animal Crossing: New Horizons, but unfortunately, both rely heavily on chance and require a lot of time and patience.

1. Around the beginning of the Summer season in June for the Northern Hemisphere and December for the Southern Hemisphere, you should hear from Isabelle in the morning announcements about the new seasonal crafting materials and recipes. If you haven't already learned how to make the Shell Wreath DIY Crafting recipe, she will send it to your NookPhone during the announcements. There has been a lot of confusion in the community about when and how this morning announcement from Isabelle occurs, but based on our current knowledge, it can only occur when there are no other notable announcements.
2. During the Summer season, all Balloon Presents that fly over your island have a chance of dropping one of nine different Summer Shell DIY Crafting recipes! When you see one, simply pull out your Slingshot and shoot it. Unfortunately, there is no way to predict exactly what you will receive as a balloon gift, so you will have to just keep shooting them down until you get lucky. Duplicate crafting recipes are also easily possible and should be expected, so we recommend attempting to trade with friends in order to complete the collection as quickly as possible. You may also be able to recoup a significant amount of money by selling any duplicates on the internet.