In the list 'Mother's Day', 'Easter', 'Mardi Gras', and 'Pancake Day', the odd word out is 'Pancake Day'.
Reasoning: Mother's Day, Easter, and Mardi Gras are significant holidays with established traditions and often religious or cultural importance. Pancake Day (Shrove Tuesday) is a more informal celebration primarily associated with culinary traditions before Lent and does not hold the same level of broad cultural or religious significance as the others.