When is the best time to buy a plane ticket?

I’m trying to buy tickets from DCA (DC) to SAN (San Diego) for a weekend in October (flexible weekend). I’m wondering when is the best time to buy tickets for my trip. I’ve been checking everyday on Kayak and I’ve heard that the best time to buy cheap tickets is on Tuesday afternoons. Is this true? Also, what would be the cheapest fare for a ticket from DCA to SAN?