What's the most efficient way get the first day of the current month?

If you don't mind including ActiveSupport in your application, you can simply do this:

require 'active_support'
date = Date.today.beginning_of_month

Time.parse("2009-10-26").strftime("%Y-%m-01")

require 'date'    
now = Date.today
Date.new(now.year, now.month, 1)