The Eighth Day is a solemn gathering immediately after the Festival Booths.

This is a reach. In Catholicism Octave means an eight-day celebration, that is, the prolongation of a feast to the eighth day (dies octava) inclusive. The feast itself is considered the first day, and it is followed by six days called “days within the octave.” The eighth or octave day is kept with greater solemnity than the “days within the octave.”

The Easter Octave is from Easter Sunday to the Second Sunday of Easter or Divine Mercy Sunday, each day being another “little Easter.” The Easter octave “overrides” any other feasts on the calendar. Christmas also has an octave, but it is very different from Easter, because it is filled with various feast days, but yet each day is still another “little Christmas.”

It is also said that Jesus rose from the dead on the eighth day (which is why Sundays are considered on par with solemnities). So Sunday is the Eighth Day.

The Messianic Fulfillment is The Judgment where we are judged. The Christian participants in the Eighth Day will be punished severely.

All of the above are the Appointed festivals and are found in Leviticus 23.

The exception is the New Moon Sabbath which we should assume is a Sabbath for the new month. It is preceded by a Preparation Day.