Outlook cannot schedule a meeting on multiple dates that do not fit a
regular pattern. To create multiple appointments on irregular dates, you
have three options:

1) Create the first appointment, then Ctrl+drag it to the other dates to
create a copy of the appointment on each of those dates.

2) Create a recurring appointment that comes close to fitting your schedule,
then drag those appointments that are on the wrong dates to the correct
dates.

3) Use the WS:Repeat Appointment tool from
http://www.websetters.co.uk/WSAddIns/Repeat%20Appointment/Index.htm to
schedule repeating appointments that do not fit a regular pattern.

If you want to set up meetings, #1 or #3 would probably be best.