Tweets made during the working day have the most engagement

"Twitter is big. With over 140 million active users, and 340 million Tweets per day, the social network is one that brands cannot ignore. Indeed, few brands are ignoring Twitter. Our recent report with global research firm Booz and Co. shows that 77% consider Twitter a priority social media platform.
It's great that brands are active on Twitter, but it's crucial they know best practices for publishing engaging content. Reach and engament can vary drastically with minor tweaks, so between December 11, 2011 and February 23, 2012, Buddy Media analyzed user engagement from more than 320 Twitter handles of the world's biggest brands. Primary success metrics included:
Reply Rate: number of replies as a percentage of followers
Retweet Rate: number of Retweets as a percentage of followers (includes manual retweets)
Engagement Rate: a combination of the replies and Retweets in the number of followers
The result is our latest data report, "Strategies for Effective Tweeting: A Statistical Review." We encourage you to download the report and share it with your friends and colleagues. We also included a "Tweet Cheat Sheet" that you can print and hang in your office or cubicle. The report is a companion to our popular Facebook report, "Strategies for Effective Wall Posts: A Statistical Review."
Here are some of the key findings from the report:
Tweeting during the day is the way to go. Tweets during "busy hours" (8AM-7PM) receive 30% higher engagement than Tweets published outside that time frame. This includes Tweets published on Saturday and Sunday.
Keep it short and sweet, and watch people Tweet. Our study found that Tweets containing less than 100 characters receive 17% higher engagement than longer Tweets.
Use hashtags, but don't overdo it. Tweets with hashtags receive two times more engagement than those without hashtags. But going overboard has a negative impact. Tweets with one or two hashtags receive 21% higher engagement than those with three or more hashtags. Using more than two hashtags actually leads to a 17% drop in engagement."
Source:  Press release from Buddy Media, 26th June 2012