Learn Dutch: Vandaag is het de derde mei. - Today is the third of May.

Because the sentence starts with the time word Vandaag (today). In Dutch, when you put anything other than the subject at the beginning of a sentence, the verb and subject swap places. This rule is called inversion.

Why do we use het here?

Just like in English ('It is the third of May'), Dutch uses het (it) as an empty or 'dummy' subject when talking about dates, the time, or the weather.

Why isn't there a word for 'of' between derde and mei?

In Dutch, you don't use a preposition like 'of' when giving a date. You simply say the ordinal number directly followed by the month, so de derde mei literally translates to 'the third May'.

How do you form ordinal numbers like derde?

To make an ordinal number (first, second, third, etc.), you usually add -de or -ste to the regular number. For the number drie (three), it changes slightly to become derde (third). This is an irregular form you just have to memorize, much like 'three' becoming 'third' in English.