"Happy weekend" is the most common way to say "快乐的周天" in Engpsh. "Weekend" refers to the two days of the week that are typically not workdays, which in most places is Saturday and Sunday. The word "weekend" originated in the late 19th century, and it originally referred to a period of rest or recreation that extended beyond the usual workday.
Here are some example sentences using the phrase "happy weekend":
"I hope you have a happy weekend!" (我希望你度过一个快乐的周末！)
"What are your plans for the weekend? Anything fun?" (你周末的计划是什么？有什么有趣的事吗？)
"I'm looking forward to a relaxing happy weekend at home." (我期待在家里度过一个轻松愉快的周末。)