This is the 3076th most frequent Dutch word.
d
The Dutch word "d" is not a standalone word but the fourth letter of the alphabet.
'De' serves as an article preceding a singular masculine or feminine noun.
De docent gaf de student een compliment.
The teacher gave the student a compliment.
Here, 'd' is the first letter of a noun, 'Dierendag', which is a proper noun referring to a holiday.
Dierendag wordt in Nederland op 4 oktober gevierd.
Animal Day is celebrated in the Netherlands on October 4th.
In this context, 'dag' (day) contains 'd', and it is a common noun.