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

References

Skin Sales Super-Thread by Coreporeal