Riot announced a while back that they would be bringing out some of the legacy skins and make them available for purchase for a while for all summoners across all servers. The League of Legends Legacy Skin Vault contains those skins that have become outdated and have been retired.
However, that is not to say that all of these skins are of an undesirable quality. Many of them are greatly in demand and as such Riot tends to open up the legacy skin vault every now and then and trot out some of these for eager summoners willing to shell out the requisite RP.
You’ll have from November 29 to December 31 to browse and purchase the skin you want. During this period, each and every one of the skins will be coming on sale for one day at a time. It would be wise for you to take note of the sale days given below and wait for the skin to come on sale before buying it.
You can check out the list here. You can also go here to see a list of skin previews in video form. If that still doesn’t convince you, you can always check out the model of the skin for yourself on LolKing.
Keep in mind that this is different from the slew of skins coming out as part of the Snowdown Showdown, the annual League of Legends Christmas event.