I think you should give her a few days to figure out what she is going to wear, especially if she is pretty busy. It also helps build up some expectation. :) I think you're more likely to get a 'yes' if she has time to prepare properly too, since I would probably be caught off guard if I was asked to go out the same day you did the asking. It does come off as last minute, like you just sort of decided to do it on the spur of the moment. Making plans super far ahead would come off as a little odd to me if it was just a standard dinner out, but it would make sense if you were asking her to a concert or something that requires tickets or reservations. You can ask for her number first if you want, but a face-to-face is always more personal. If you are really nervous, go for the phone call though. Friday is probably the best day since she is less likely to have something huge going on the next day and you can stay out later if you two want (if she doesn't have something Saturday).