Harrowing, Championship, Older, Vaulted Skins question

In regards to skins that are not available for whatever reason, are there ever any windows in which the skins become available again? Harrowing comes to mind.. During late October (Halloween) season, does Riot put out ALL previously released Harrowing skins for purchase? Or does it remain a you missed it it's gone forever scenario?

The same question can go for event type skins.. I am not sure why Dragonblade Riven is currently unavailable.. It was released for a real world event, much like say.. Masked Shaco was, yet Shaco is still available, Riven is not. Does the Riven skin only become available for a small window once per year? Or again, is it gone forever?

I Believe I understand the Championship skin debate and agree that they are not readily available (although I wish it were not the case)..

Just wondering if any vets out there have a better understanding of the skin release / re-release than I.

Clarification please?