Someone is having a bad (or good) day, and from the events that happened to him/her, this person thinks that the universe is against (or very good with) him/her. Then, an event happens that is so good (or bad) that it seems to make this day suddenly seem to become the opposite of what it was. It could be anything like winning the lottery, his/her crush deciding to date him/her, this same crush rejecting him/her when it seemed that such a thing wouldn't ever happen, or a relative dying. What word defines this event that seems contrary to how the day initially seemed to be? I'd like the word to be an adjective but if it can't be an adjective then I guess I'll need to accept another type of word besides adjectives. The word needs to be used to say things like "What a(n) [...] event" or "This event really is/would be [...]". Thank you in advance.
A day with so many reversals of fortune might be called a day worthy of the Fates.
The concept that you want to convey is very complicated. This means that the adjective or phrase you want will almost certainly have to be drawn from literature or mythological concepts that are widely known.
You can make up an adjective related to a character in a more contemporary work, but here you have to know your audience. Even the most widely-watched TV shows have not been seen by everyone. Nor have most video games.
How about reversal?
3: a change (as of fortune) often for the worse
You could use game changer
a newly introduced element or factor that changes an existing situation or activity in a significant way.
This can be used to describe the event that changed how the day unfolded.
I woke up feeling miserable, but winning the lottery was a real game changer.