If you intend to use the same one for Advent and Christmas, you probably want something unaccompanied, yes? The Michael Haydn Lenten Masses are nice for Advent. (MH552 at cpdl, 553 at imslp, and there's another which is more austere). If you don't have parts, Mass XVII is the standard.
Christmas? We use Yon Mass of the Shepherds, which is borderline cheesy but pretty easy. If your group is better than my group, consider the Missa de Nativitate Domini, Op.44 (Klein, Bruno Oscar) (women or mixed with organ) or Grand’Messe de Noël 'Puer natus est' (Gevaert, François-Auguste) (SSA org) (both imslp). Mass VIII is a tradition.
Yes, I made some assumptions here. So did you. Don't make assumptions, and you'll get better-targeted info.
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