What to do about Holy Innocents falling on a Sunday…. Who gets the Divine Service? It seems we probably need to do what any Lutheran normally would do: run to the Anglican sources!
In all of these sources, there are no propers given for a mass on Holy Innocents’ Day. In Lossius and Bezeli there ARE special propers for Matins & Vespers (midday Offices also in Bezeli). There is also a Vesperale & Matutinale by Ludecus where I expect I will also find Daily Office propers.
Certainly these editors must have foreseen the possibility for the 28th to fall on a mass day. (In Magdeburg 1613, there were Divine Services held on Tuesday & Thursday in addition to Sunday- an even higher chance!) The lack of Holy Innocents propers suggests that the Sunday in the Octave of Christmas would take precedence and be the source of propers, even if it fell on the 28th. (This is not to say that the Sunday after Christmas had all of its own propers even. Bezeli repeats the introit from Christmas on St. Stephen, St. John, and the first Sunday after Christmas, while Ludecus repeats only on the Sunday, providing new proper introits for the two saint days)
As far as 16th & early 17th century Lutheran Kantors were concerned, if there was a mass to be sung on the 28th, it wouldn’t be for Holy Innocents.