By the last day everybody is sunburned, out of clean clothes, and living inside that specific ache of something about to end that cannot be extended.
Sunday is when the festival stops competing with itself. The afternoon belongs to a completely different energy, no more sprinting between conflicts, no obligations left, just the crowd watching itself on the grass while Gobi and Sonora pull the most devoted audiences of the weekend. Then the closing headliner goes on late, near midnight, and hands the crowd an emotional conclusion nobody knew they were owed. This is the day that has produced the sets people still argue about years later, the ones that outgrew the festival and became their own cultural events.
The polo fields in Indio, the final day of the first weekend, camping through the night.
This is the one you describe to the friend who has never been. Start with how quiet the drive back is.