I absolutely love mashed potato, with loads of butter and milk and a little mustard! How about you?
I love mashed potatoes but never heard of them with mustard. You usually use cream, garlic and butter.
The kind of mustard I use is the wholegrain variety. It really adds a nice hint of flavor. Garlic is really good too. Gosh, you could put anything in mash and I'd love it!
Yeah, you really see people use it around the holidays like Thanksgiving and Christmas. I would use it with other dishes too 
A Japanese friend of mine had no clue how to make mashed potatoes, so I had to teach her. Seemed strange, as I grew up with that food.
the dish started appearing in 1747 with an entry in "The Art of Cookery" by Hannah Glasse. Dehydrated and frozen mashed potatoes are available in many supermarkets.
Mashed potato may be used as an intermediary ingredient for other dishes such as dumplings and gnocchi, in which case the potatoes may be baked or boiled, and may or may not have dairy or seasoning added.
Ingredients.
The use of "floury" types of potatoes is recommended, although "waxy" potatoes are sometimes used for a different texture
. There are a multitude of "floury" types, but the most commonly known include russet, golden wonder, and red rascal potatoes. Butter, vegetable oil, milk and/or cream are usually added to improve flavour and texture, and the potatoes are seasoned with salt, pepper, and any other desired herbs and spices. Popular ingredients and seasonings include: garlic, cheese, bacon bits, sour cream, crisp onion or spring onion, caramelised onion, mayonnaise, mustard, horseradish, spices such as nutmeg, and chopped