This Sunday is called New Sunday for three reasons:

Firstly, Easter means freedom and the new Sunday is a day of double freedom. God has set us free from sins, and however we are under the influence of passions. And when God does everything in a new way order to set us free from needs, it is called New Sunday.

This Sunday is called New Sunday also because of our resurrection. On the first Sunday Christ raised us from the dead thanks to His Soul and Power, and on the second Sunday he will raise us from the dead bodily, so that raising from the dead we will become immortal.

And finally, this Sunday is called New Sunday because we celebrate the feast of our soul saving in spiritual joy and fete.