Use ordinal numbers to express the date. First you name the day, then the month, then the year. Days are always masculine, meaning they are preceded by a masculine definite article. In written German, the day and year are always given in numerals.

Examples:
Heute ist der 31. April. = spoken as: der einunddreißigste April
Mein Geburtstag ist der 17. März 1950. = spoken as: der siebzehnte März neunzehnhundertfünfzig

If you want to mention the day of the week, it precedes the date and is separated by a comma.

Example:
Heute ist Mittwoch, der 31. April = spoken as: Mittwoch, der einunddreißigste April

More:

The months can be referred to by their names or as ordinal numbers in written and spoken German. January is the first month of the year, February is the 2nd, and so on. Since this is much shorter to write, you'll see it often in business correspondence and e-mails.

Example:
der 31.4. = spoken as: der einunddreißigste Vierte