Learn Russian: Какое сегодня число? Пятнадцатое июля. - What is the date today? The fifteenth of July.

When asking "what is the date", Russians literally ask "which date". Because the word for date or number (число) is a neuter noun, we must use the neuter question word Какое to match its gender.

I thought число meant "number". How does it mean "date" here?

You are right! Число literally means "number", but it is also the standard word Russians use for the calendar date. So asking Какое сегодня число? literally translates to "Which number is it today?".

Why does Пятнадцатое (fifteenth) end in -ое?

Ordinal numbers (like first, second, fifteenth) act like adjectives in Russian. Even though the word число isn't repeated in the answer, Пятнадцатое still takes the neuter -ое ending to match it. You are essentially saying "Пятнадцатое число" (the fifteenth number).

Why is July written as июля instead of its dictionary form июль?

To say "the fifteenth OF July", Russian uses the Genitive case. Since июль is a masculine noun ending in a soft sign (-ь), we drop the soft sign and add -я to make it июля.