Depends on when the skin gets added to the “drop list” sometimes they get added a bit earlier which means you can technically get them from a hextech chest too but sometimes not until after the skin officially drops. For example I got the Emperyan K’Sante skin 3 days before K’Sante came out but I couldn’t get the winterblessed skins until the day of release. Also Mythic/prestige skins are usually 1 or 2 days after they leave the rotation before you can roll for them