"Sometimes, when you have a hard day, something good happens, and you may end your day with happiness."
Your second sentence sounds a little incorrect. Below, i wrote a sentence that you maybe wanted to write. Now I'll try my best to explain where is the mistakes in yours. I am not a proffy, but don't underestimate me reading a lot in English) firstly, you wrote "although it was a tough day" and with no doubt it means you are talking about one certain day. Secondly, you wrote " we can finish the day with happiness, if something happy was happened in the end". This two parts of your sentence don't go well with each other, because second part isn't about something that happened once. The word "if" says you that "something happy" may not happen in reality. I suggest you to learn about conditions(like first, second condition and etc) also there is a grammar mistake in your second part. It's about present and past perfect( present perfect is when " have happened" and past perfect when "had happened"). If I am right you were supposed to use present perfect according to the part where you have written "we can finish" where "can" is used in present form. If you have used "could" then only you could have used "had happened". And lastly, you better not use alike words like "happiness" and "happy" near each other. I hope I helped you)