Timing is only one aspect. If you are a novice, try to do as much "homework" as you can about renovation even before you meet up with them. Aim to equip yourself with the knowledge to evaluate their competency, proposals etc, and for the one you select, to be able to supervise them effectively. Such knowledge cover many things including choice of materials, product/brand knowledge, pricing/value comparison etc. The more you know, the less likely things will go wrong.
Also, don't forget that there are many things you need to do that has nothing to do with your ID/contractor. My approach is to get all these sorted out beforehand so that when renovation starts, I'm focused on monitoring/supervising their work instead of being busy trying to figure which aircon model to get, which installer to engage and where to get the best deal