Cleo's sixteenth birthday is established as Apr 19, 2006. Since it is fairly self-evident that the first few full moons in the series are consecutive (the first two certainly are), some dates can be implied by back-tracking full moons. This would make Cleo's "siren" experience to be Fri Apr 14, 2006. Neil Gilbert's 50th birthday and Emma's moon-fever experience, Wed Mar 15, 2006. If that date is correct, then it is proven that Cleo, Rikki and Emma became mermaids under the full moon of Mon Feb 13, 2006; (actually the following morning).