Freeze windows and honest calendars
United Kingdom calendars are crowded with freezes that engineering sometimes treats as optional footnotes: retail peaks, payroll cycles, year-end close, regulated reporting windows, board demos that cannot absorb a login outage. A migration plan that ignores them is a plan that will be rewritten under stress.
We ask sponsors for hard no-go dates before we propose a weekend. If the only free window sits beside a payroll run, we say so early. Moving the plan is cheaper than discovering the conflict after vendors have booked people.
Honest calendars also protect relationships. Operations teams remember who scheduled a cutover against their busiest night. Put freeze windows on the same page as dependency maps — they are constraints, not decorations.