Hi PANuccia,
Technically it 's not wrong but I would avoid it here, let alone for a formal/semi formal email. Perhaps it's a matter of taste but it comes across as being a little banal, wishy-washy, blah. There are so many other adjectives to use instead.
Have an enjoyable, relaxing, good, great weekend etc
If someone told me to have a nice w/e I'd probably respond with a "I'm going to be aiming higher than that!"
