I am a Scrum Master for a team that runs a 3 week sprint followed by 1 week of regression testing. It is becoming a more common practice for the developers to pick up stories or technical debt in the regression week (once all stories for the current sprint have been developed and are being regression tested). The regression week tends to be a quite week for the developers who mainly do bug fixes that are found in regression.
Having the devs pick up new stories/tech debt in the regression week means these are ready to be tested on the 1st day of the next sprint.