Originally, this quote seems to have come from hunting but it's since morphed into all sorts of applications.
In my situation it means that sometimes I have a great day and sometimes it's not so great. I particularly like the Yin Yang/ do or do not binary aspect on offer. Seemingly opposite, the two states are actually interconnected. There is no one without the other.
In my Al Ain days, my Arabic teachers had their own version - Yom asal, yom basal, which means ‘One day honey, one day onions’.
Same basic principle applies - one day you're winning and one day you're scoring own goals.
I tend to use these expressions in hindsight. Rarely during a day, and never ever at the start of a day.
At the start of every day I expect to eat the bear!
As Groucho said: