Sorry, it's been a SINGLE fucking day.
Pete!
Ah, Pete, a very amusing grammatical swap of SINGLE and LONG. However, I believe the sentence would better give the required emphasis if the qualifier "fucking" were placed before the adjective, rather than the noun.
"It's been a fucking long day" specifically stresses the ennui of the day length as opposed to "It's been a long fucking day" which might indicate an over-extended period of procreational activity.
What we need on this website is a grammatical pedant.