someone/SHIELD plans an important meeting/mission/thing for the 14th February and is checking everyone is available to attend when Clint (or Natasha, or person of your choice) points out that, no, because it's Valentine's Day and they have plans. And everyone laughs, some because 'yeah, right', others because obviously that's not as important as this Very Important Thing, and Clint (or whoever) is all: no, seriously. My partner would kill me.