MotherinIsrael and Zeeskite: Sorry for making it ‘sound’ confusing, but Avram in MD and Am Yisrael Chai both explained it well.
There are in fact EIGHT notes in an octave. Otherwise it would be called a septave had there been only seven notes in it.
‘Oct’ is 8 as in octagon or octogenarian etc.
An octave is the interval between two notes where the higher note has twice the frequency of the lower note. On a piano keyboard, this corresponds to an interval of 8 white notes eg C D E F G A B C.
We do not say that a week has 8 days cos it starts and ends on a ??? or a Sunday, so to be logical the music scale seems to have only 7 notes. But when you come to play a musical scale, it sounds incomplete unless you play all 8 notes starting with the lower note and ending with the higher one, as AYC correctly pointed out from do-re-mi- ending again with –do.