From the shop Teewit4grownups Funny T-Shirts and Gifts:

Tired of ketchup getting all of the attention? Mustard lovers unite! Show off your affinity for the world’s best condiment with this funny “I Put Mustard on My Mustard” design.
Sizes: S,M,L,XL,2XL,3XL.
Colors: Ash Grey,Natural,Light Blue

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August 2nd is National Mustard Day.  What better way to celebrate that with a Mustard shirt.  And sure enough, there are plenty of Mustard shirts to make you happy.  Here is just one of the great mustard shirts available.  Have a laugh and make sure you have a hot dog with that mustard.