For the purpose of transparency, when does Scrum say a new increment of working software...

According to the Scrum Guide1, an Increment is a concrete stepping stone toward the Product Goal. Each Increment is additive to all prior Increments and thoroughly verified, ensuring that all Increments work together. In order to provide value, an Increment must be usable. Multiple Increments may be created within a Sprint. The sum of the Increments is presented at the Sprint Review thus supporting empiricism. However, an Increment may be delivered to stakeholders prior to the end of the Sprint. The Sprint Review should never be considered a gate to releasing value. For the purpose of transparency, when does Scrum say a new increment of working software must be available? At the end of every Sprint.

References: Scrum Guide