Most of chapter 29 deals with the seventh, and most sacred month of the Hebrew year (so Ashley, page 568), when there were three celebrations: New Year Festival, Day of Atonement, and Festival of Shelters. Verses 1-6 deal with the offerings at the New Year Festival and are largely parallel to Lev 23.23-25. Translators may begin a new paragraph here with a subheading (see the introductory comments on this section). Good News Translation‘s heading for these six verses is “The Offerings at the New Year Festival.” Contemporary English Version has “The Sacrifices at the Festival of Trumpets” (see the comments below).
On the first day of the seventh month you shall have a holy convocation: The seventh month of the Hebrew calendar corresponds to mid-September to mid-October. The Hebrew pronoun for you is plural, referring to the Israelites. For holy convocation, see 28.18.
You shall do no laborious work: See 28.18.
It is a day for you to blow the trumpets is literally “It will be for you a day of a shout [or, blast].” The Hebrew word for “shout/blast” is the same one rendered “alarm” in 10.5. The Hebrew text here lacks the word for trumpets, so La Nouvelle Bible Segond and Traduction œcuménique de la Bible render this sentence as “It will be for you a day of acclamation.” Bijbel: Vertaling in opdracht van het Nederlandsch Bijbelgenootschap is similar with “It will be a day of jubilation for you,” and so is Parole de Vie with “it is the day of shouts of joy.” Trumpets were blown during the New Moon Festivals and the other religious festivals (see 10.10), but the ram’s horn became the instrument of choice for the New Year Festival (so Plaut, page 1084). So another possible model for this sentence is “It will be a day for you to blow rams’ horns” (similarly New Jewish Publication Society’s Tanakh, De Nieuwe Bijbelvertaling). In the post-biblical period this day became the celebration of the Jewish New Year (also called “Rosh Hashanah”).
Quoted with permission from de Regt, Lénart J. and Wendland, Ernst R. A Handbook on Numbers. (UBS Helps for Translators). Miami: UBS, 2016. For this and other handbooks for translators see here .