The list is ordered from the oldest to the most recent announcement made by the Riot staff regarding skin sales.

Pre-Seasons Seasons
Pre-Season One Season One
Pre-Season Two Season Two
Pre-Season Three Season Three
Pre-Season Four Season Four
Pre-Season Five Season Five


Period Sales Link
September 13
2011
September 16
2011
PantheonSquare.png Ruthless Pantheon [S|L] - 50% off
KayleSquare.png Viridian Kayle [S|L] - 50% off
GragasSquare.png Gragas, Esq. [S|L] - 50% off
[1]
September 16
2011
September 19
2011
KarthusSquare.png Phantom Karthus [S|L] - 50% off
MalzaharSquare.png Vizier Malzahar [S|L] - 50% off
BrandSquare.png Apocalyptic Brand [S|L] - 50% off
[2]
September 20
2011
September 23
2011
UdyrSquare.png Black Belt Udyr [S|L] - 50% off
KatarinaSquare.png Bilgewater Katarina [S|L] - 50% off
NasusSquare.png Dreadknight Nasus [S|L] - 50% off
[3]
September 23
2011
September 26
2011
VayneSquare.png Vindicator Vayne [S|L] - 50% off
KassadinSquare.png Pre-Void Kassadin [S|L] - 50% off
LuxSquare.png Sorceress Lux [S|L] - 50% off
[4]
September 27
2011
September 30
2011
ShenSquare.png Blood Moon Shen [S|L] - 50% off
GangplankSquare.png Spooky Gangplank [S|L] - 50% off
RammusSquare.png Molten Rammus [S|L] - 50% off
[5]
September 30
2011
October 3
2011
SorakaSquare.png Dryad Soraka [S|L] - 50% off
VladimirSquare.png Count Vladimir [S|L] - 50% off
CaitlynSquare.png Safari Caitlyn [S|L] - 50% off
LeBlancSquare.png Prestigious LeBlanc [S|L] - 50% off
[6]
October 4
2011
October 7
2011
MalphiteSquare.png Shamrock Malphite [S|L] - 50% off
Twisted FateSquare.png Tango Twisted Fate [S|L] - 50% off
WarwickSquare.png Big Bad Warwick [S|L] - 50% off
[7]
October 7
2011
October 10
2011
Lee SinSquare.png Acolyte Lee Sin [S|L] - 50% off
IreliaSquare.png Nightblade Irelia [S|L] - 50% off
VeigarSquare.png White Mage Veigar [S|L] - 50% off
OlafSquare.png Glacial Olaf [S|L] - 50% off
TeemoSquare.png Recon Teemo [S|L] - 50% off
[8]
October 11
2011
October 14
2011
RumbleSquare.png Rumble in the Jungle [S|L] - 50% off
BlitzcrankSquare.png Boom Boom Blitzcrank [S|L] - 50% off
CorkiSquare.png Hot Rod Corki [S|L] - 50% off
[9]
October 14
2011
October 17
2011
AkaliSquare.png Blood Moon Akali [S|L] - 50% off
UrgotSquare.png Butcher Urgot [S|L] - 50% off
Miss FortuneSquare.png Waterloo Miss Fortune [S|L] - 50% off
WarwickSquare.png Tundra Hunter Warwick [S|L] - 50% off
Xin ZhaoSquare.png Viscero Xin Zhao [S|L] - 50% off
SingedSquare.png Surfer Singed [S|L] - 50% off
[10]
October 18
2011
October 21
2011
JaxSquare.png Jaximus [S|L] - 50% off
SingedSquare.png Hextech Singed [S|L] - 50% off
GarenSquare.png Desert Trooper Garen [S|L] - 50% off
[11]
October 21
2011
October 24
2011
SwainSquare.png Northern Front Swain [S|L] - 50% off
MorganaSquare.png Sinful Succulence Morgana [S|L] - 50% off
ShenSquare.png Frozen Shen [S|L] - 50% off
[12]
October 25
2011
October 31
2011
Cho'GathSquare.png Nightmare Cho'Gath [S|L] - 50% off
KarthusSquare.png Grim Reaper Karthus [S|L] - 50% off
YorickSquare.png Undertaker Yorick [S|L] - 50% off
[13]
October 28
2011
October 31
2011
OriannaSquare.png Sewn Chaos Orianna [S|L] - 50% off
WarwickSquare.png Feral Warwick [S|L] - 50% off
FiddlesticksSquare.png Spectral Fiddlesticks [S|L] - 50% off
[14]
November 1
2011
November 4
2011
LuxSquare.png Commando Lux [S|L] - 50% off
AniviaSquare.png Noxus Hunter Anivia [S|L] - 50% off
SorakaSquare.png Divine Soraka [S|L] - 50% off
[15]
November 4
2011
November 7
2011
Miss FortuneSquare.png Road Warrior Miss Fortune [S|L] - 50% off
AniviaSquare.png Hextech Anivia [S|L] - 50% off
NasusSquare.png Galactic Nasus [S|L] - 50% off
[16]
November 8
2011
November 11
2011
SkarnerSquare.png Earthrune Skarner [S|L] - 50% off
NidaleeSquare.png Pharaoh Nidalee [S|L] - 50% off
UdyrSquare.png Primal Udyr [S|L] - 50% off
[17]
November 11
2011
November 7
2011
Twisted FateSquare.png High Noon Twisted Fate [S|L] - 50% off
CassiopeiaSquare.png Siren Cassiopeia [S|L] - 50% off
NasusSquare.png Pharaoh Nasus [S|L] - 50% off
[18]
November 15
2011
November 18
2011
VeigarSquare.png Baron Von Veigar [S|L] - 50% off
LeBlancSquare.png Wicked LeBlanc [S|L] - 50% off
OlafSquare.png Forsaken Olaf [S|L] - 50% off
[19]
November 18
2011
November 21
2011
LeonaSquare.png Valkyrie Leona [S|L] - 50% off
SionSquare.png Warmonger Sion [S|L] - 50% off
KayleSquare.png Unmasked Kayle [S|L] - 50% off
[20]
November 22
2011
November 25
2011
NocturneSquare.png Ravager Nocturne [S|L] - 50% off
Kog'MawSquare.png Caterpillar Kog'Maw [S|L] - 50% off
ZileanSquare.png Groovy Zilean [S|L] - 50% off
[21]
November 25
2011
November 28
2011
The Legacy Skin Project:
NunuSquare.png Sasquatch Nunu [S|L] - 75% off
EzrealSquare.png Nottingham Ezreal [S|L] - 75% off
PoppySquare.png Blacksmith Poppy [S|L] - 75% off
EvelynnSquare.png Masquerade Evelynn [S|L] - 75% off
GragasSquare.png Scuba Gragas [S|L] - 75% off
SivirSquare.png Spectacular Sivir [S|L] - 75% off
Dr. MundoSquare.png Mr. Mundoverse [S|L] - 75% off
Dr. MundoSquare.png Toxic Dr. Mundo [S|L] - 75% off
KennenSquare.png Swamp Master Kennen [S|L] - 75% off
SionSquare.png Hextech Sion [S|L] - 75% off
ShenSquare.png Yellow Jacket Shen [S|L] - 75% off
AnnieSquare.png Red Riding Annie [S|L] - 75% off
NidaleeSquare.png Leopard Nidalee [S|L] - 75% off
MordekaiserSquare.png Dragon Knight Mordekaiser [S|L] - 75% off
Kog'MawSquare.png Sonoran Kog'Maw [S|L] - 75% off
TwitchSquare.png Kingpin Twitch [S|L] - 75% off
RyzeSquare.png Professor Ryze [S|L] - 75% off
MalphiteSquare.png Shamrock Malphite [S|L] - 75% off
TryndamereSquare.png Highland Tryndamere [S|L] - 75% of
AmumuSquare.png Pharaoh Amumu [S|L] - 75% off
TeemoSquare.png Badger Teemo [S|L] - 75% off
[22]

References

Skin Sales Super-Thread by Coreporeal