The Visperad is not read or recited independently. It exists to expand the Yasna at the great seasonal festivals of the Zoroastrian calendar, the gahambars, and at a handful of other high occasions, and its own structure only makes sense laid over the Yasna it accompanies. Its name comes from vispe ratavo, all the masters, and its content is mostly invocation: it addresses a wide roster of ritual authorities and sacred categories that the plain Yasna does not name individually.
In practice a priest performing an expanded ceremony inserts the Visperad's twenty three karde, or sections, at fixed points within the Yasna's own sequence rather than reciting the two texts one after another. The result is a single combined liturgy in which the Visperad thickens the invocatory sections without altering the Yasna's core structure, including the Gathas embedded within it. Because it has almost no independent narrative or doctrinal content of its own, the Visperad is best understood as a set of insertions rather than as a companion scripture, and its interest to a reader depends on already knowing the Yasna it exists to extend.
The gahambars themselves mark the six seasonal creations Zoroastrian cosmology recognizes, sky, water, earth, plants, animals and humanity, so an expanded Visperad ceremony is tied to specific points in the agricultural and pastoral year rather than performed on an arbitrary schedule.