The ascent is the lead-up, when we already feel the air of what awaits us. The peak is the yom-tov itself. The descent is the days following the festival, when we still feel the festival mood, especially on Is’ru Chag, the day immediately after the yom-tov.
The name Is’ru Chag is from Psalm 118:27, part of Hallel, which reads, Is’ru Chag ba’avotim ad kar’not hamizbe’ach – “Bind the festive offering with cords to the altar”. Others translate the verse, “Order the festival procession with boughs, up to the horns of the altar”.
Tradition gave the Hebrew words the figurative meaning, “Bind (hold on to) the festival” so that the festive atmosphere lingers.
In the Jerusalem Talmud the day after a yom-tov is b’reh d’mo’eda – “the son of the festival”, which conveys the same idea.
There is a link with Shavu’ot, in Israel a one-day festival: in Temple times certain offerings were brought on the following day, which gave it a festive character. In addition, on other festivals the meat of the offerings could still be eaten on the next day, adding a special flavour to the descent from the peak.
No important occasion should be allowed to be quickly forgotten. It should brighten and influence the days ahead. The same with a person who was important in one’s life. Even after they are gone their example and influence should linger.