When it comes to official birthdays, there is no true definition of when the St. Louis Blues were formed. You could go with their first official game, which was October 11, 1967.
You could go with the day the league held the expansion draft. Or, you could just as easily go with April 5, 1966.
The reason that day could be considered the birthday of the Blues is that is the day they officially became the 12th team of the NHL. That is the day they kept Baltimore from becoming a franchise in the NHL as well.