The planning period involved all key stakeholders and buy-in. This includes QA, engineering, and the customer.
In a GxP validated release, everything starts from the documentation and ends with a sign off by the QA team before it's delivered to the customer who does their own validation process. So it was absolutely critical to get agreement between the teams.
I was Director of Engineering and represented the engineering team. Over a 1-2 week period, the stakeholders would meet to discuss the objectives and we'd draw up a rough outline ("Business Requirements"). Then each lead would take that rough outline back to their teams to determine feasibility (for the customer team, it was to rank priority so if we came back to make cuts, we knew what to cut). For engineering, we'd review each feature and breakdown to determine if we could do it all given our resource load and time frame ("Technical Design Specification"). We'd provide a rough idea of how we would implement it and consult with the QA team on how they would test it ("Functional Requirements Specification" and "Validation Plan") and if there was some angle we missed.
But because a lot of this ended up being negotiations and documentation ("Here's what you want, here's how it'll look, here's how we'll build it, here's how we'll test it, these are the compromises we have to make for now, this sub-feature will be released next cycle"), I ended up doing most of that "dirty work" and only consulted the team when needed and to get their buy in. This left my team with plenty of time to play with that new framework or build a sandbox for some new tooling or build process and so on.
Even if you are a skeptic, I strongly recommend reading the Shape Up guide. It's very practical and prescriptive.