Don't allow any sacred cow to keep you from eliminating items from your calender and "to do" lists. This sounds strange. Sacred cows don't normally do that.
Cows are sacred to Hindus.
sacred cow - Definitions from Dictionary.com
Stop thinking or believing in such a sacred cow! Yes, that's a good way to use it.
2. Yes it's a common expression. Anything that is taboo, that you aren't allowed to question in your society (but which is still doubtful) is a sacred cow.