The most significant Holiday in Christianity, the Passion of our Lord Jesus Christ and his Glorious Resurrection after 3 days in his grave, i.e. Easter, is celebrated (in the Western Church) on the first Sunday after the first full moon in spring. This year, spring (or Nowruz in Farsi, the Iranian New Year) starts today, the vernal equinox. And since the full moon is seen tomorrow, Easter will be on March 23 (while the Good Friday is tomorrow).
So, Happy Nowruz! for all my Iranian friends, and Happy Easter! for the others.