1) The date the attacks were conducted was religiously significant:
The Paris attacks came on the first day of the Muslim lunar month of Safar, which coincides with the anniversary of the Prophet Mohammed’s first successful “ghazva” (raid) against the “infidel” at Safwan in 623 AD.
2) The killers may have been hopped up on a type of amphetamine popular in the Middle East. That's speculation rather than fact at this point — we'll have to hear what the French drug lab says.