They talk of Christmas so long that it comes.

People can spend so much time discussing an approaching occasion that the occasion arrives while they are still talking about it. The proverb draws attention to the gap between anticipation and experience.

It can also gently tease people who keep returning to the same future event. Their talk has not brought it about; time has simply passed as the conversation continued.

Use it when an upcoming occasion has been discussed repeatedly and is now close at hand or has arrived. It works as a light observation about prolonged anticipation, rather than a claim that talking about something makes it happen.