Setting your working hours gives your colleagues an idea of when you are (and aren't) available for meetings. You cannot customize the hours for each day in Outlook, so for those working part-time or who have irregular schedules, I recommend setting your working hours to encompass any possible working time, particularly during the weekdays. For example, if you work Mondays from 7 AM to noon and Wednesdays from 3 PM to 7 PM, you would set your working hours to Mondays and Wednesdays from 7 AM to 7 PM. You can then block off time on your calendar to mark the hours you aren't available on each specific day (see Setting Up Working Elsewhere).
To set your working hours:
In Outlook, click on File.
Under Calendar, enter your Start time and End time, and check the boxes for the days of the week that you work.