Can I lock a race day or a day in the past?

A race day cannot be locked. In fact a race day offers no per-day controls at all — no padlock, no pencil (Training Day Rules), no “I cannot run on this day” and no “Let TrainAsONE plan this day”. The race owns the day, so to change it you edit or delete the race itself.

See How is a race day shown in my training plan?.

If a race day already holds a lock from before — for example one that fell inside a rolling lock — the padlock switch does appear so you can release it; you simply cannot create a new lock on it. And a race day inside a rolling lock is still shown as a race day, never as a locked day.

A day in the past cannot be locked either. Locking is about future planning, so past days have no padlock.

The same applies to a day whose run is already behind you: once a run on a day has been counted and analysed, that day no longer offers the per-day controls, so there is nothing left to freeze or change.