The Invitatory consists of Psalm 95 (Veníte, exsultémus Dómino), Psalm 100 (Iubiláte Dómino, omnis terra), Psalm 67 (Deus misereátur nostri et benedícat nobis) or Psalm 24 (Dómini est terra et plenitúdo eius). They are sung to very specific psalm tones, which can be found in the Liber Hymnarius.
The antiphons that go with the Invitatory psalm can also be found in the Liber Hymnarius.
An example: antiphon Christus natus est nobis (LH 16) with Psalm 95 (tone IV**):
(I think I'll cut myself a *little* slack, here, though. As I look at the various sources that are available to me -- online and in books -- there is a lot of mismatch amongst them. I'm sure there is a complicated explanation for it all, but I'm just going to go with what's in the LH. Thanks again.)
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