Didn't find a schedule but here is what NBA.com says about it :
"If first-round series are completed in six games or less, the conference semifinals may move up to begin on either Saturday, April 30 or Sunday, May 1."
And Wojnaroswki says in his latest article "Boston needs to sweep the series on Sunday and give themselves a full week of rest before Game 1." so I guess it will either start on Saturday or on Sunday.