It's a common question, and one that doesn't have a straightforward answer. The when to buy car insurance Martin Lewis really depends on your individual circumstances.
If you're buying a new car, then you'll need to insure it from the moment you drive it off the forecourt. However, if you're simply renewing your existing policy, then you may have some flexibility in when you do so.
The most important thing is to make sure that you're covered from the start of the new policy year. That way, if anything happens during that year, your insurer will foot the bill.
Of course, there are other considerations too. If you're about to go on a long trip or make any major changes to your life (like getting married), then it's worth checking with your insurer to see if there are any implications for your cover.
In short, there's no definitive answer to the when should I buy car insurance Martin Lewis question - it all depends on your individual situation. Just make sure you're covered from the start of the new policy year and you should be fine.