If I understand what you are asking, the simple answer is:
When did we decide to do something?
However, it is true that that is ambiguous: it could be asking about when we made the decision, or when the activity was to happen.
Often the ambiguity won't matter, as we are much more likely to be asking about the decision for the future than the process of making the decision; but it could be an issue.
I cannot think of a systematic way of disambiguating. I would probably ask
When was it that we were going to do something?
but that loses the explicit "decide".