If you have a blog already that focuses on one topic but have an urge to focus on another topic, that is when you write a new blog. I find that having separate blogs that talk on specific topics keeps them consistent as far as the reader is concerned.
The best time for a new blog is that time when you are ready to write, when you have time to set it up and dedicate the hours, days, weeks and months necessary to get it going and keep it going. The best time for a new blog is the time that you choose, and no other time.
if you are wanting to do it, do it! XD I wouldn't launch it after just one post though, I'd say do a few posts first (I did about 10) but its personal preference really, but just so people have a few posts by you to read :D