I know in Classical Latin, dies (day) was masculine, except for special days, which were feminine. I thought that when the Christians switched from that old lingua franca, Aramaic to that other old lingua franca, Latin, they decided that every day was special, or something like that, because you get stuff like from the Dies Irae where it's feminine, e.g., dies irae, dies illa. But our little community chorus is doing a Te Deum and, in the Salvum Fac Populum Tuum section we get the line: Dignare, Domine, die isto sine peccato nos custodire.

So my question is:

Does anyone know from whence the text of the Te Deum? Or is this day not so special?