It depends which variant of English you're using, and it depends whether you're speaking or writing. If you're using British English, you can say either 'the ninth of April' or 'April the ninth'. They are both equally correct. We don't often write the 'the' or the 'of', though. When you're writing, you can put either 9th April or April 9th. American English only uses the second option: April 9th, read as 'April ninth'.