Errr, even if it did occur in Muharram, the month had no significance. The Nabi (saw), Imam Ali (as) and Bibi Fatima (sa) found out about what will happen in Muharram, in Karbala on Imam Hussain's birth through Jibraeel. They were not aware. I'll look for a hadith to back this up InshaAllah.
interesting point ... can you tell me a bit more about this ... just for extra knowledge?