Affordable Option: Topman, $65
Aspirational Option: A.P.C., $250

Yes, it does get cool enough during the summer months to have a sweater in the rotation. Just make sure it's a thin knit sweater made with lighter fabrics. A nautical sweater with breton stripes is a classic item that dates back to the origins of the French navy, and one reason it's stuck around so long is its versatility. Layer up with this for a late night bonfire or a rooftop BBQ and you will not be that sucker in the corner who can't stop shivering in the middle of July.