Question about spelling in spanish
why is correct to say "buenos dias" instead of " bueno dia". Also, why is dia masculant?
3 Answers
If you want to wish someone a good day (1) you say "buen día." When "bueno" is used as an adjective preceding the noun, the O is dropped. It is not dropped when it follows the noun. In the case of "good day," it´s just not common to say it with the adjective following the noun even though it would be grammatically correct.
Welcome to the forum, Cesar.
Your gender question is a common one, and the plural use has been asked a number of times as well. It's good to get in the habit of doing a forum search. At the top of the Answers pages, just above the start of the threads, is a search box. By doing a search you can see lots of answers and opinions by different people.
It is funny how it's plural, isn't it? In other languages, "good morning", "good afternoon", and "good night" are often singular! 
Just because "día" ends in an "a" doesn't make it feminine.
