To call someone on Tues at an unspecific time:
If this is an easy person to reach and I can see no real stumbling blocks I would put call Mrs. X on Tuesay on my date book with no time or if I think a certain time is good, I would put it in for that time as well as an untimed entry (because these are at the top of myl ittle Palm's screen). I would verify that I have her phone number and I might copy it to Tuesday and put next to her name. If my day seems to be getting crowded, I might also give this an appt. time.
If this is super, super important and there is no fall back time, and the person can be hard to reach and my life is such that calling can be hard, I might take specific actions to ensure that I have phone access on Tuesday, and can receive a call back. If this is a phone appt and there is any chance of a mistake, I might call or e-mail the Friday before or on Monday to verify that she is expecting my call and to make sure that she has my various numers just in case I can't reach her. If there are time zone issues I would try to determine them and if so, I might make this a timed appt. on the calendar.