People today don't put a lot of store in rules of etiquette. Life has become a little bit looser, a little more freeform. But when you ask someone out on a date, you still want to give them plenty of notice. Traditionally, it is considered bad manners to call any later than Thursday for a Saturday date (Wednesday for Friday). This may seem antiquated, but it shows a certain level of respect for the person you are interested in. Once you have been dating for a while and are considered a 'couple,' this maxim drops away and the two of you can make your own rules.