An Etymological Dictionary of the German Language/Messe
Messe, f., ‘mass, fair,’ from MidHG. mësse, misse, f., ‘mass, church festival, fair’ OHG. mëssa, missa, f. So too the word on which it is based, MidLat. missa, signifies not only ‘incruentum christianorum sacrificium,’ but also ‘feast of a saint’ (“quod in eo Missa sollemnis peragitur”). The latter sense led to MidLat. missa, MidHG. mësse, ‘fair,’ because this “was wont to be held on account of the great concourse of people” on saints' days (comp. Fr. foire, ‘fair,’ lit. ‘holiday,’ under Feier). MidLat. missa, “as is well known, originated in missa est, scil. concio, the words spoken by the deacon when dismissing the congregation which did not partake of the sacrament”; from this the corresponding Rom. cognates, Ital. messa and Fr. messe, are derived. The vowel in AS. mœsse, f. (Northumbr. messe), E. mass, is abnormal; the latter also signifies feast in Christmas and Lammas (see Laib). Comp. Feier, Mette, Nonne, Opfer, and Vesper.