You didn't say what model you have so I am assuming that the grass comes out the right side of the deck. Which means you have right hand thread, which is the normal direction. you will probably need either an impact wrench or something to hold the blade and then put on a good wrench and a cheater bar.
I have seen a case where the blade bolt went all the way through the spindle with a nut on top, that was installed finger tight and no wrench used. Come back in a month later that requires a 1" 1600 lb/ft impact to remove the nut. the 3/4 900 lb/ft impact wouldn't touch it.