So for example, every Sunday I need to do a big grocery shop. It doesn't need to be done at a specific time and could theoretically could be done on Friday or Sat, but Sun is usually the best day. Where to track that? A checklist might make sense, but let's say someone invites me to do something on Sunday that takes up most of my day. I look at my calendar and say, sure let's do it but then forget that I need to my shopping. This is a little bit of a silly example but you get the idea.As you mentioned, you've said it before and are now saying it again... So, can you give me an example of a routine you have that doesn't work in your GTD system? Maybe that will help the other GTDers and I answer your question better.
So this gets back to my question about routines and checklists, when do you check them (whether scheduling or deciding on what to do next)?