That's the claim of two American applied mathematicians, who analysed words used in 2.4 million internet blogs such as Twitter, giving a score out of nine depending on how positive they were.
Professors Christopher Danforth and Peter Dodds, of Vermont University, gave words like free, fun, rainbow and sex 8+ score, while words such as betray, cruel, hatred and suffocate all scored less than two.
Saturday and Sunday rated predictably well but, most shockingly, Monday came out as the second happiest day of the week, partly because people were still reminiscing about the weekend, reports The Telegraph.
However by Wednesday, these feelings had subsided.
Professor Danforth said: "People's daily experience is being reflected somewhat in what they're saying. And this is more of a story, we think, for Twitter messages, which we've just started to pay attention to, about a year ago.
"They think they are communicating with friends, but since blogs are public we're just looking over their shoulders.
"The weekends tend to be fairly happy and Wednesday turned out to be the saddest day."
The expert added: "What we're attempting to do is measure collective happiness on a much larger scale, similar to measuring the temperature outside.
"The energy of a few molecules bouncing around doesn't give a good indication of heat, you need billions or more."