  We construct a generalization of the Day convolution tensor product of
presheaves that works for certain double $\infty$-categories. Using this
construction, we obtain an $\infty$-categorical version of the well-known
description of (one-object) operads as associative algebras in symmetric
sequences; more generally, we show that (enriched) $\infty$-operads with
varying spaces of objects can be described as associative algebras in a double
$\infty$-category of symmetric collections.
