Legendary Snowball Question

I just bought Prime time Draven (which is a Legendary skin), but I didn't get my free mystery skin. Am I supposed to get it at a later, random time or am I supposed to get it right after?

"From December 23 until January 7 at 23:59 PST, whenever you buy or are gifted a Legendary or Ultimate skin, you’ll get a free Mystery skin. If that Mystery skin is a Legendary or an Ultimate, you’ll get another Mystery skin and so on. As an added bonus, we’re doubling your chance to unlock a Legendary when you receive each Legendary Snowball Mystery skin."