"Buenos días" is the more formal and common greeting. It literally translates to "Good mornings" and is used in most situations, especially when addressing someone you don't know well or someone older than you.
"Buen día" is considered more informal and literally translates to "Good day." It's often used in casual conversations with friends, family, or people you know well.
However, both greetings are widely accepted in Mexican Spanish, and the choice often depends on personal preference and the specific situation.
Here's a quick breakdown:
* Formal: Buenos días
* Informal: Buen día
It's always best to err on the side of formality when unsure. Using "Buenos días" will never be considered rude, while using "Buen día" might be perceived as disrespectful in certain situations.