You can find both the chant and Vaughn Williams versions in The English Hymnal, circa 1936. You will also discover enough verses for a real Procession for each of the major feast - Easter, Ascension, and Pentecost - as well as another for the Dedication of a church. If you're looking for Latin, "Analecta Hymnica" has versions ranging fro 6 to 20+ verses for all the major Feast of both Jesus and Mary, and even some of the major Western saints. I have used the Vaughn Williams version a lot in the past. There is a version in "Songs of Praise" (from OUP, edited by RVW and Percy Dearmer) for regular Sundays. And I have authored a Doxology verse specifically for the musical verse with triplets.
'Salve Festa Dies' is also in the old Solesmes book "Processionale Monasticum" dating from 1893, but reprinted by Solesmes. This slim book has processional chants for most Sundays of the year and many major feasts. It's listed by Paraclete Press (I just looked).
Another useful book of chants (not propers) for use throughout the church year is "Cantus Selecti". (Unfortunately, it doesn't contain Salve Festa Dies).
If anyone still needs a source for this, it's on p. 237 of Laudes Festivae (1940; ~10MB pdf), and p. 1928 of Mass and Vespers (1957; ~112MB pdf), both available from this site.
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