According to the Watford Observer Evanna and Katie will be doing an autograph signing:

Two cast members from the fifth Harry Potter film will visit Tesco Extra, Watford, on Saturday, July 14.

Katie Leung, who plays Cho Chang, and Evanna Lynch, who play Luna Lovegood, will be singing autographs between 1pm and 3pm.

A host of other activities inspired by the world’s most famous wizard will take place at the store in Lower High Street, Watford, from 11am to 5pm.

Thanks EvannaOnline!
Okay, back to work for me. Ha ha.