Officially I'm UC (sometimes it's been indeterminite colitis though), but I want to chime in that much the same is the case with me. As far as good days go, or stretches of good days, I find I normally transition slowly from bad to good, very gradually, when I am lucky enouhg to do so at all, but that a bad day can come up on me just like that.
Most typically I worsen in terms of pain, during the night in my sleep (usually waking me up due to pain). In general I suffer more pain at night and on wakeup, and my best times tend to be just before bed. This might be partly due to taking pred (when I do take it) mostly in the a.m., but even in those stretches of being pred-free I have noticed the worst pain in late morning and on 'wake-up' (again, if in alot of pain I have usually been up alot during the night).
Pancolitis dx'd 1986, full med-free remission 88-97
Flaring or simmering ever since
10 20 mg pred, 100 mg Imuran
Probiotics (mainly Primal Defense)
Turmeric/circumin, various vitamins/minerals,
Low-carb version of Specific Carb Diet