the best time is when the child is ready.
Not when the parent is ready.
Big difference.
The other posters here gave good advice... and I ditto them.
It will either be a battle (because the child is not ready, but the parent expects them to be) and then it will be a LONG drawn out thing. I'm talking even 1 year long. Or, it will be a happy good learning thing for them, because they ARE ready.
think of it this way... a child may know in their head's what something is and "how" to do it. It does not mean they CAN do it. Same for adults.... we may understand how a rocket is built and how it flies... but that does NOT mean that WE can then build a rocket from scratch, even with the instructions. Some adults don't even know how to bake a cake even with a recipe. Even if we mentally understand it.
For a child, they may know in their heads "how" to dribble a basketball... but that does NOT mean they can actually DO it, nor proficiently, nor without fumbling around and making mistakes. Right?
Well, same for potty training. It takes practice and patience to get it right. Even if they do get it right, they will STILL have accidents. Even a Pro Basketball player, makes mistakes. So, for a mere child and pottying... they WILL have accidents, regressions, and mistakes. This has to be accepted and allowed. Otherwise.... it will be a real stressful thing for the child... because the parent will not have their expectations met about it.
Sure, get a potty chair, books, whatever. But this is only to introduce it. Not having her to learn it and do it now. She is still SO young.
Sorry for rambling... but pottying is such a stressful thing or a battle often times... because the parental "expectations" of it is not in line with the child's capacity for actually doing it.
So, the best time to start pottying is... is when the child is ready.
All the best,
Susan