Spoiler for episode 100 

"We already know that Samantha Smith will return as Mary Winchester in ‘Supernatural’s 100th episode. Can you tell us more about the other characters they are bringing back? – Leticia
Actually, Smith is returning not for the milestone outing, but for the episode airing two weeks prior, on April 1 (no joke). No. 100 will offer longtime fans a couple blasts from the past, but it’d spoil all the fun if I ID’d them for you. Just pay close attention come April 15."