generally used for hypothetical, doubtful, or wished-for circumstances. Not in this case. There is an elipsis: [te deseo] que tengas un buen día. ([I wish you]

generally used for hypothetical, doubtful, or wished-for circumstances. Not in this case. There is an elipsis: [te deseo] que tengas un buen día. ([I wish you] that you have a good day). Edit: OK, I answered too fast. Its use in this case indeed indicates wished-for.