Hello
The following is perfectly correct: Τι μέρα είναι σήμερα; (what day is today?)
An appropriate answer to the above is: Δευτέρα (Monday).
This is a different, but also perfectly correct question: Πόσο του μηνός έχουμε σήμερα;
An appropriate answer to the above is: Δώδεκα (twelve). Meaning this is the 12th day of the month.
Although the word "πόσο" is widely used in the above expression, the grammatically correct phrase would be "Πόσες του μηνός έχουμε σήμερα;"
We can use both έχουμε and είναι. So the following is correct: Πόσες του μηνός είναι σήμερα;
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Vasiliki Baskou, Instructor/Director, https://learn-greek-online.com.