I rarely ever have what I'd categorically call a "bad day." The trick for me is to see life in full color: don't think in black-or-white terms. Don't let isolated negative events color your whole day. Each day is a diverse banquet of various experiences, but sometimes we'll tend to let a few consecutive instances of bad luck glom together in our mind as one monolothic overlay of suckitude. Try to let isolated instances remain discrete little events; a handful of many that fill your day.
"How could we forget those ancient myths that stand at the beginning of all races, the myths about dragons that at the last moment are transformed into princesses? Perhaps all the dragons in our lives are princesses who are only waiting to see us act, just once, with beauty and courage. Perhaps everything that frightens us is, in its deepest essence, something helpless that wants our love."
-- Rainer Maria Rilke, Letters to a Young Poet