The word "all" is a generalization that indicates the people as a whole came together. AT: "The people gathered all together" (See: [[rc://en/ta/man/translate/figs-hyperbole]])
This would have been all or part of the first five books of the Old Testament.
# On the first day of the seventh month
This is the seventh month of the Hebrew calendar. The first day of the seventh month is near the middle of September on Western calendars. AT: "On day 1 of month 7" (See: [[rc://en/ta/man/translate/translate-hebrewmonths]] and [[rc://en/ta/man/translate/translate-ordinal]])
# brought the law
"brought The Book of the Law"
# all who could hear and understand
This would include children who were old enough to understand what was being read.