$\mathrm{\infty }$-operads via symmetric sequences

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