It is all in the males chromosomes. They have both the X and the Y. We only have the X - So if you don't get your boy you can always blame it on Hubby. LOL
I don't really think any of the calendars or other tales really can determine what gender child you are having. Like Bethany said - it's fun to look at but not accurate at all.
Everyone told me I was carrying a girl because of my belly size and shape, or something (this was 14 years ago so my memory is vague). I had a boy.
Whichever gender you have, you know you will that child with all your heart and that is all that matters.