I have been playing a College of Glamour bard in the Tomb of Annihilation module. They get the Enthralling Performance feature at 3rd level when they choose the subclass (Xanathar’s Guide to Everything, p. 14):
If you perform for at least 1 minute, you can attempt to inspire wonder in your audience by singing, reciting a poem, or dancing. At the end of the performance, choose a number of humanoids within 60 feet of you who watched and listened to all of it, up to a number equal to your Charisma modifier (minimum of one). Each target must succeed on a Wisdom saving throw against your spell save DC or be charmed by you. […] This effect ends on a target after 1 hour, if it takes any damage, if you attack it, or if it witnesses you attacking or damaging any of its allies.
If a target succeeds on its saving throw, the target has no hint that you tried to charm it.
According to the description, the target does not become aware of the fact that someone attempted to charm them if they succeed at their saving throw. However, it doesn’t say anything about what happens if they fail their save and the charm wears off.
Once the effect of Enthralling Performance wears off, do those that were previously affected by it know they were charmed?