Attacking a variation of the Vigenere with very long key length
The important part about the length is how many letters appear on those 17 characters:
YVBCXGJRYHHRCJIUL
WEDNESDAYTHURSDAY
WEDNESDAYSATURDAY

We have the plaintext of 12 of the 17 characters. This should probably let us recover 12 character of the key, and then guess the remaining characters based on context (key is probably made of words, not randomly).