Which one should you use?
There is a distinction between asking about the past and the present. While como foi o seu dia? asks how the day was, the phrase como vai o seu dia? asks how the day is currently progressing.
Is there a formal and an informal way to say it?
The choice of greeting changes the register. Oi is a more common and casual way to say "hello" than olá in daily Brazilian Portuguese conversation. For a more informal tone, you can say E aí, como foi o teu dia? or combine a greeting like Tudo bem? with a question such as Tudo bem? Como foi o teu dia?
How does it change in a sentence?
In the phrase como foi o seu dia?, using the word seu is the standard way to address someone with a level of distance or respect or in a neutral and formal context.
When do people say it?
Brazilians tend to use more physical contact during greetings, such as hugs or cheek kisses. A common way to respond to a question about your day is to say Foi bom (It was good) followed by e o seu? (and yours?).
Did you know?
Example conversation
Lucas greets Juliana when she arrives home.
- LucasOlá, como foi o seu dia?Hello, how was your day?
- JulianaFoi ótimo, e o seu?It was great, and yours?
- LucasO meu foi bem cansativo.Mine was very tiring.
- JulianaSinto muito, Lucas.I am sorry, Lucas.