Formal:
* Jak minął Pani/Panu dzień? (How did your day go?) - This is the most polite and formal way to ask.
* Jak się Pani/Panu dzień mijał? (How was your day?) - This is also formal, but slightly more casual.
Informal:
* Jak Ci minął dzień? (How did your day go?) - This is the most common way to ask among friends and family.
* Co u Ciebie? (What's up?) - This is a very informal way to ask about someone's day, similar to "What's up?" in English.
You can also use more specific phrases depending on the context:
* Co porabiałeś/aś dzisiaj? (What did you do today?)
* Miałeś/aś dobry dzień? (Did you have a good day?)
* Co się dzisiaj działo? (What happened today?)
Remember to adjust the pronoun (Pani/Pan/Ci) depending on the gender of the person you are speaking to.