If you place Month on your rows shelf and then right-click + drag your day field to the text shelf of your marks column. Choose the Count Distinct option. Is your data divided so that you have month names in columns and you have day numbers in columns? It would probably be preferable to use data that had an actual date stamp. This would allow you to make line continuous between years, trend lines, do other types of analysis and it allows you to do these calcs if you have multiple years.