I was looking at the dates a while back and like other "occult crimes" (e.g. Son of Sam, Zodiac, etc) I think that there is something to them, but exactly what I cannot be totally sure.
But, what I noticed was that Jack the Ripper committed his crimes on the following dates:
1) Nichols--August 31
2) Chapman--September 8
3) Stride & Eddowes--September 30
4) Kelly--November 11
3) 9+30=39 (again)
So what I noticed was that these dates include numbers that have been associated with occult philosophy for a long time now. For instance, both 1 and 3, when you add the numbers total 39 and this number according to mysticism (e.g. Kabbalah, Masonry etc) has traditionally held a certain occult, even sinister, significance to it. For no. 4 you need only google the numbers together to find out a wide array of opinions as to its meaning (especially after 9/11). So I'm sure there's more to the picture than meets the eye
but I definitely believe that it's no coincidence he--or they--chose the dates that he did to commit his killing spree.