Many tools exist that tell you when are the best times to tweet. The analysis is usually based on the number of tweets sent by your followers, as an indication to when there are most people online. Tools that do that include tweriod, tweet o’clock, timely, etc. My service welshare shows you the currently tweeting followers. But I have to argue that the above assumption is not entirely correct. The best time to tweet is not when most of your followers are tweeting, but right before that. In the most active periods of the day the stream becomes too noisy, and it gets harder to get noticed. While if you send your tweet right before everyone appears online, they will see it before the stream gets filled with noise.
It is of course hard to tell when is that time, because people don’t appear online all at the same time. But since everything in this metric is an approximation anyway, you can subtract 30-60 minutes from the estimated peak of online presence of your followers, and here’s your suggested time to tweet.