The potatoes sold at Gregori's from August 14th to the 16th (including those purchased by restaurants and the Ministry's cafeteria) have been tainted! People who eat these potatoes won't notice any taste difference, and they don't look any different, but after eating them (at any point on Tues, Wed, Thurs) they will feel inexplicably happy. Even if they have the worst luck, or find out that a loved one has died, they will remain happy, giggly, and giddy. The effects will wear off 36 hours after eating the potato dish, and if someone had mashed potatoes for lunch then french fries for dinner, the effect will be compounded.
The Ministry will spend the next 2 weeks looking into the matter, finding it the result of several bored farmers' wives who didn't realize the effect would spread throughout their gardens.