Sunday is rather special [excpe a noted for major feasts like Christmas, ect.]
Look at the practice of the Liturgy of the Hours. There is Monday Morning Prayer and Monday Evening Prayer. This is the same Monday through Friday. It follows our normal method of counting time as if it was from midnight to midnight.
However, come Saturday there is only Saturday Morning Prayer, no Saturday Evening Prayer. What you have in its place is Sunday Evening Prayer I. On Sunday you have Sunday Morning Prayer and* Sunday Evening Prayer II*
**
Sunday is the Lord’s day and every Sunday is a ceebration of Easter and as such it is a feast day. I always think of it as there is no Saturday eveing in Liturgical time and as Resurrection people we celebrate Easter for really long day.