"Have a nice weekend!"
I respectfully disagree.
You can say either Hezký víkend. = '(Have a) nice weekend.' or Přeji ti/vám/vám hezký víkend. = 'I wish you (sg informal/sg formal/plural) a nice weekend.'
Mějte hezký víkend doesn't sound right. It's an anglicism (a word/expression borrowed from English) and although you can sometimes hear it in spoken Czech, it's something that is very sub-standard and it should not be used, uttered or even thought of. :)