I think an important aspect is separating "tech debt time" completely from your backlog or planning. Generally speaking, there's less stakeholders involved in r

I think an important aspect is separating "tech debt time" completely from your backlog or planning. Generally speaking, there's less stakeholders involved in reducing tech debt and less need for requirements gathering or prioritization - developers generally know where the worst problems are and can be relied upon to choose something to focus on next. I'd avoid the temptation even to have "projects" during that time - make it purely about accomplishing refactors and tech debt removal that can be completed during that day, and specifically don't have plans or a backlog.