The following is the list of all the "festivity" champion skins in the client.

Festivities Name Skins
Lunar New Year / Lunar Revel
Valentine's Day
Carnival / Masquerade
Saint Patrick's Day
April Fools' Day
Easter
Halloween / Harrowing
Christmas / Winter Holidays/ Snowdown Showdown
Football / FIFA World Cup / Legends of the Field
Winter Olympics