Attacking a variation of the Vigenere with very long key length

Question

I am to decrypt the ciphertext YVBCXGJRYHHRCJIUL into its plaintext message. The information that I am given is that the key used in the cipher is very long (equivalent to the length of an entire book) and that the key is certainly longer than the message being encrypted. The only other information I know is that the message encrypted is 2 days of the week, such as "SATURDAYWEDNESDAY."

There is no way to do any statistical analysis to discover key length. I am completely lost as to how I am to accurately decrypt the ciphertext into the correct message.

EDIT: It is known that the days of the week are "in order" and so based on the length of the ciphertext I can determine that it must be WEDNESDAYTHURSDAY or WEDNESDAYSATURDAY. This will reveal to me the first 9 characters of the key, but if the key is never repeated then I cannot use this to decrypt the second half of the ciphertext.

Answer 1

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).