There is probably no fixed standard, and I am not sure there is any authority that might set one. I believe many Latin speakers do not leave out the anno.
However, when Pope Benedict XVI announced his resignation in Latin, he did use this precise format (no mensis, no anno), and as luck would have it, he read it out loud. (There are better versions, but I chose this one because it has Latin subtitles.)
The text in question is:
… declaro me ministerio Episcopi Romae, Successoris Sancti Petri, mihi per manus Cardinalium die 19 aprilis MMV commisso renuntiare ita ut a die 28 februarii MMXIII, hora 20, sedes Romae, sedes Sancti Petri vacet …
So he was made Pope on 19 April 2005 and resigned effective 28 February 2013.
Here is what I hear:
die undevicesimo aprilis bis millesimo quinto
die vicesimo octavo februarii bis millesimo [muddled] tredicesimo
I believe he bungles the second year. It should be bis millesimo tertio decimo, but I think he stumbles and eventually comes out with tredicesimo, which does indeed mean “thirteenth,” but not in Latin – I believe his Holiness slipped into Italian here.
For 28, one might also rather have said duodetricesimo, but that is besides the point.
So the answer to your question, at least according to the previous head of the Catholic church, would be:
- mensis and anno are not introduced in the spoken form
- the month is in the genetive
- the year is in the ablative