To say ‘Have a lovely day’ in German, you would say ‘Haben Sie einen schönen Tag!’ if you are using the formal ‘you’, or ‘Hast du einen schönen Tag!’ for the informal ‘you’.
This phrase captures the sentiment of wishing someone a pleasant day. The word ‘schönen’ means ‘lovely’ or ‘nice’, and ‘Tag’ means ‘day’. It’s a warm and friendly expression that you might use when parting ways with someone.