It is well known that the two alleluias cited in the title are textings of the same tune – indeed they are only two of many textings of that tune. Along with the alleluia Video celos, these two were sung over a rather short space of time in the liturgical year: on the feasts of Christmas, St Stephen's and Epiphany. It happens that for another purpose, involving the transmission of chant in later manuscripts, I have collated the alleluia Dies sanctificatus and alleluia Vidimus stellam in over a hundred manuscripts, most of them late French sources. The choice of these manuscripts rather than others – French rather than German or English – was dictated in part by practical considerations, but also by the conviction that French sources are on the whole richer in variants and hence more revealing in the mechanics of their transmission. The approximately one hundred sources considered are a good majority of the surviving late sources, and the picture they give is, I believe, representative. In the course of this article, I will also have occasion to mention earlier manuscripts, both French and non-French. These are not the primary focus of my study, and therefore I have permitted myself many omissions in that respect. Important graduals, for example from St-Omer,2 Angers3 and Noyon,4 to name only three, have been untouched. My collation has shown a considerable number of variants, some of which are clearly regional in nature, while others are of a more ambiguous character. Having two textings of the same melody available supplies a useful control: readings that occur in both versions naturally have a strong claim to represent a true tradition, rather than a random alteration.