sounds about right, there will of course be warnings and updates telling us when to buy oru skins by etc. usually a week or bit less/more after the date- ie october 31st or december 25th.
CHRISTMAS GIFT IDEA: why not give all summoners a coupon for a skin of thier choice, under the maximum limit of 975rp, (so you cant snipe free legendaries, also dissallow teh buying of limited edition content with this "coupon of sorts") for christmas?
that would be the greatest gift of all riot, and i'm sure would just give us mroe incentive to give back to you!!
edit: so people can't use it to too much fo theri advantage if you put like a 2 week cap on it it ensures users need to buy somethign already out, and cant wait half a year for some amazing new skin you folks will no doubt create.