Last date to submit an application for coverage to start 1st of next month ?

In general terms what is the last date of the previous month to submit an application so that coverage starts the 1st of the new month within enrollment period and outside of enrollment period ?  I am trying to submit an application for my wife for coverage to start April 01, 2015. So what is the last date to submit an application in March 2015 ?  She has a qualifying event outside of enrollment period and is looking to select the Blue Choice Gold  PPO Plan.