Isaiah 66:23 - Ellicott's Commentary On The Whole Bible

From one new moon to another... — Under the Mosaic law Israelites were bound, at least in theory, to attend the temple at the three great feasts. In the new Jerusalem, as the prophet thought of it, the pilgrimages would be both more frequent and more universal. Every sabbath and new moon would witness not Israel only, but “all flesh,” thronging into the courts of the temple. It lies in the nature of the case that the words never have received, and never can receive, a literal fulfilment. The true realisation is found in the new Jerusalem of Revelation 21:22-27, of the perpetual sabbatism of Hebrews 4:9, and even that glorious vision is but the symbol of spiritual realities.