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
Pre-Season Six Season Six
Pre-Season Seven Season Seven
Pre-Season Eight
Early Sales


Period Sales Link
November 20
2012
November 23
2012
ShacoSquare Royal Shaco [S|L] - 50% off
RenektonSquare Rune Wars Renekton [S|L] - 50% off
UdyrSquare Primal Udyr [S|L] - 50% off
[1]
November 23
2012
November 26
2012
IreliaSquare Frostblade Irelia [S|L] - 50% off
AsheSquare Amethyst Ashe [S|L] - 50% off
Miss FortuneSquare Cowgirl Miss Fortune [S|L] - 50% off
[2]
November 27
2012
November 30
2012
GravesSquare Jailbreak Graves [S|L] - 50% off
VarusSquare Blight Crystal Varus [S|L] - 50% off
GarenSquare Rugged Garen [S|L] - 50% off
[3]
November 30
2012
December 3
2012
DariusSquare Lord Darius [S|L] - 50% off
TalonSquare Renegade Talon [S|L] - 50% off
Cho'GathSquare Jurassic Cho'Gath [S|L] - 50% off
[4]
December 3
2012
December 7
2012
DianaSquare Dark Valkyrie Diana [S|L] - 50% off
LuluSquare Bittersweet Lulu [S|L] - 50% off
NunuSquare Demolisher Nunu [S|L] - 50% off
OlafSquare Glacial Olaf [S|L] - 50% off
EzrealSquare Frosted Ezreal [S|L] - 50% off
[5]
December 7
2012
December 10
2012
ViktorSquare Full Machine Viktor [S|L] - 50% off
CassiopeiaSquare Desperada Cassiopeia [S|L] - 50% off
SingedSquare Augmented Singed [S|L] - 50% off
[6]
December 11
2012
December 14
2012
Searing Seeds:
ZyraSquare Wildfire Zyra [S|L] - 50% off
WarwickSquare Firefang Warwick [S|L] - 50% off
WukongSquare Volcanic Wukong [S|L] - 50% off
[7]
December 14
2012
December 17
2012
Ice Fort War:
JannaSquare Frost Queen Janna [S|L] - 50% off
MalphiteSquare Glacial Malphite [S|L] - 50% off
AnnieSquare Frostfire Annie [S|L] - 50% off
[8]
December 18
2012
January 6
2013
Opening the Legacy Skin Vault - Legacy skins previously removed from the store:
HeimerdingerSquare Alien Invader Heimerdinger [S|L] - 50% off
JaxSquare Angler Jax [S|L] - 50% off
AnnieSquare Annie in Wonderland [S|L] - 50% off
TeemoSquare Badger Teemo [S|L] - 50% off
PoppySquare Blacksmith Poppy [S|L] - 50% off
UrgotSquare Butcher Urgot [S|L] - 50% off
AkaliSquare Crimson Akali [S|L] - 50% off
GarenSquare Desert Trooper Garen [S|L] - 50% off
MordekaiserSquare Dragon Knight Mordekaiser [S|L] - 50% off
TaricSquare Emerald Taric [S|L] - 50% off
WarwickSquare Feral Warwick [S|L] - 50% off
TristanaSquare Firefighter Tristana [S|L] - 50% off
ShenSquare Frozen Shen [S|L] - 50% off
SionSquare Hextech Sion [S|L] - 50% off
TryndamereSquare Highland Tryndamere [S|L] - 50% off
TwitchSquare Kingpin Twitch [S|L] - 50% off
NidaleeSquare Leopard Nidalee [S|L] - 50% off
EvelynnSquare Masquerade Evelynn [S|L] - 50% off
AlistarSquare Matador Alistar [S|L] - 50% off
Dr. MundoSquare Mr. Mundoverse [S|L] - 50% off
Cho'GathSquare Nightmare Cho'Gath [S|L] - 50% off
EzrealSquare Nottingham Ezreal [S|L] - 50% off
PoppySquare Noxus Poppy [S|L] - 50% off
KarthusSquare Phantom Karthus [S|L] - 50% off
AmumuSquare Pharaoh Amumu [S|L] - 50% off
RyzeSquare Professor Ryze [S|L] - 50% off
CorkiSquare Red Baron Corki [S|L] - 50% off
AnnieSquare Red Riding Annie [S|L] - 50% off
CaitlynSquare Safari Caitlyn [S|L] - 50% off
NunuSquare Sasquatch Nunu [S|L] - 50% off
GragasSquare Scuba Gragas [S|L] - 50% off
MalphiteSquare Shamrock Malphite [S|L] - 50% off
Kog'MawSquare Sonoran Kog'Maw [S|L] - 50% off
SivirSquare Spectacular Sivir [S|L] - 50% off
KennenSquare Swamp Master Kennen [S|L] - 50% off
Twisted FateSquare The Magnificent Twisted Fate [S|L] - 50% off
ZileanSquare Time Machine Zilean [S|L] - 50% off
Dr. MundoSquare Toxic Dr. Mundo [S|L] - 50% off
KayleSquare Unmasked Kayle [S|L] - 50% off
MalzaharSquare Vizier Malzahar [S|L] - 50% off
ShacoSquare Workshop Shaco [S|L] - 50% off
ShenSquare Yellow Jacket Shen [S|L] - 50% off
[9]
December 21
2012
December 24
2012
FioraSquare Nightraven Fiora [S|L] - 50% off
SorakaSquare Divine Soraka [S|L] - 50% off
AhriSquare Midnight Ahri [S|L] - 50% off
[10]
December 25
2012
December 28
2012
Save Snowdown!
VolibearSquare Northern Storm Volibear [S|L] - 50% off
CaitlynSquare Arctic Warfare Caitlyn [S|L] - 50% off
KennenSquare Arctic Ops Kennen [S|L] - 50% off
[11]
December 28
2012
December 31
2012
Beach Bust:
SingedSquare Surfer Singed [S|L] - 50% off
TeemoSquare Panda Teemo [S|L] - 50% off
RumbleSquare Rumble in the Jungle [S|L] - 50% off
[12]
December 31
2012
January 4
2013
CassiopeiaSquare Mythic Cassiopeia [S|L] - 50% off
EzrealSquare Explorer Ezreal [S|L] - 50% off
GragasSquare Gragas, Esq. [S|L] - 50% off
[13]
January 4
2013
January 7
2013
KassadinSquare Harbinger Kassadin [S|L] - 50% off
MalzaharSquare Overlord Malzahar [S|L] - 50% off
Cho'GathSquare Loch Ness Cho'Gath [S|L] - 50% off
[14]
January 8
2013
January 11
2013
TristanaSquare Guerilla Tristana [S|L] - 50% off
TeemoSquare Recon Teemo [S|L] - 50% off
CorkiSquare Urfrider Corki [S|L] - 50% off
[15]
January 11
2013
January 14
2013
Skin Super Sale:
SonaSquare Guqin Sona [S|L] - 50% off
TalonSquare Dragonblade Talon [S|L] - 50% off
Lee SinSquare Dragon Fist Lee Sin [S|L] - 50% off
WukongSquare Jade Dragon Wukong [S|L] - 50% off
VayneSquare Heartseeker Vayne [S|L] - 50% off
FizzSquare Fisherman Fizz [S|L] - 50% off
RivenSquare Battle Bunny Riven [S|L] - 50% off
MorganaSquare Blackthorn Morgana [S|L] - 50% off
SwainSquare Tyrant Swain [S|L] - 50% off
AmumuSquare Sad Robot Amumu [S|L] - 50% off
ShyvanaSquare Darkflame Shyvana [S|L] - 50% off
PantheonSquare Glaive Warrior Pantheon [S|L] - 50% off
[16]
January 15
2013
January 18
2013
XerathSquare Scorched Earth Xerath [S|L] - 50% off
GalioSquare Hextech Galio [S|L] - 50% off
ShenSquare Surgeon Shen [S|L] - 50% off
[17]
January 18
2013
January 21
2013
HeimerdingerSquare Piltover Customs Heimerdinger [S|L] - 50% off
CorkiSquare Hot Rod Corki [S|L] - 50% off
ZileanSquare Groovy Zilean [S|L] - 50% off
[18]
January 22
2013
January 25
2013
TalonSquare Crimson Elite Talon [S|L] - 50% off
RivenSquare Crimson Elite Riven [S|L] - 50% off
DravenSquare Soul Reaver Draven [S|L] - 50% off
[19]
January 25
2013
January 28
2013
TaricSquare Armor of the Fifth Age Taric [S|L] - 50% off
MordekaiserSquare Infernal Mordekaiser [S|L] - 50% off
NasusSquare Dreadknight Nasus [S|L] - 50% off
[20]
January 29
2013
February 1
2013
Common Criminals:
GragasSquare Vandal Gragas [S|L] - 50% off
VladimirSquare Vandal Vladimir [S|L] - 50% off
TwitchSquare Vandal Twitch [S|L] - 50% off
TwitchSquare Vandal Twitch [S|L] - 50% off
BrandSquare Vandal Brand [S|L] - 50% off
[21]

References

Skin Sales Super-Thread by Coreporeal