Let It Snow is related to the seasonal world event, the Feast of Winter Veil. Completing this achievement is a criterium for the meta achievement .
is a reward from performing
/kiss on a Winter Reveler, found in most inns. Other possible rewards are and . The cool down for the kiss is one hour. As Handful of Snowflakes does not bind, it is possible to buy them off other players or the auction house, or transfer them from alts.
Your best bet on finding all of the required combos is to simply hover around Dalaran.