From new moon to new moon, and from sabbath to sabbath means simply from one month to the next at the New Moon Festival and from one week to the next on the Sabbath. It stresses that there will be an endless round of rituals. For new moon and sabbath, see the comments on 1.13. Revised English Bible expresses these two lines clearly with “Month after month at the new moon, week after week on the sabbath,” and so does Good News Translation with “On every New Moon Festival and every Sabbath.”

All flesh shall come to worship before me means people from everywhere will come to worship Yahweh. This is very much an eschatological or end-time vision. All flesh refers to people from every nation (see verses 16 and 18). The Hebrew verb rendered worship is literally “bow down” (Bible en français courant; see the comments on 2.8). Before me implies the people will come to the Temple in Jerusalem to worship Yahweh.

The quote frame says the LORD emphasizes that this promise comes from Yahweh.

For the translation of this verse consider the following examples:

• From one New Moon Festival to the next,
and from one Sabbath observance to the next,
everyone will come to worship me,”
says the LORD.

• At every New Moon celebration and on every Sabbath,
every human being will come and worship me,”
says the LORD.

Quoted with permission from Ogden, Graham S. and Sterk, Jan. A Handbook on Isaiah. (UBS Helps for Translators). New York: UBS, 2011. For this and other handbooks for translators see here .