Or Sunday in the Octave of Easter; Easter is one day spread over eight, as it were, a continuous celebration (and even more so in the Orthodox rites). The newly baptised would originally wear white all the octave (the one garment the put on at baptism, so maybe not so white anymore).
Also called St Thomas Sunday, because of the gospel, and Quasimodo Sunday, which you can look up on Wikipedia; it’s a pun.
Music: Alleluial, Ye sons and daughters of the Lord (which I have never heard sung).
Flowers: yes, white, daffs, etc.