i understand that paid vacation is a benifit.Question is, Can the boss always be the one to write down at any time on the calender when that will be?

I know that there are reasons why a boss can say no,this is a bad time you can''t,but i really would like to be the one to ask when i can,because who knows better about when things are going on in my life than I !! if it''s bad time so be it.I suppose if it is legal for them to say when or not to do a vacation,They have the right,but is it really legal for them to write it on the calender and not me? My boss decides when she will take hers,and she has a boss. I know stupid and illegal are not the same thing,but can she really do this????