On the first day there shall be an holy convocation. After the Paschal meal on the evening of the 14th of Abib, there was to be a solemn assembly of the people on the next day for religions worship. The name "convocation;" applied to these gatherings, seems to show that originally the people were summoned to such meetings, as they still are by the muezzin from the minarets of mosques in Mahommedan countries, and by bells from the steeples of churches in Christian ones.
And on the seventh day. On the 22nd of Abib—the seventh day after the first holy convocation on the 15th (see Le Exodus 23:4-8). Only two of the Jewish festivals were of this duration—the feast of unleavened bread, and the feast of tabernacles (Le 23:39-42).
The Christian Church has adopted the usage for Christmas, Easter, Ascension, and Whitsuntide, where the last day of the week is known technically as "the octave." No manner of work shall be done in them.
Festival-days were in all countries days of abstention from the ordinary business of life, which could not conveniently be carried on conjointly with attendance at the services, meetings, processions, etc; wherein the festival consisted.
But absolute cessation from all work was nowhere strictly commanded except among the Hebrews, where it appears to have been connected with the belief in God's absolute rest after the six days of creation.
The command here given was solemnly repeated in the law (Le Exodus 23:6 - 8).