This video explains why we should declare the new moon of the aviv when we know we will be able to bring a sheaf of aviv barley to the priesthood 15-21 days after the first crescent sliver of the new moon is seen from the land of Israel. It also explains why, if we do NOT do it that way, it will cause the farmers with the earliest ripening barley to lose their crops. (Updated version v2, dated 24 April 2021.)
Video 1: