Hi Benetta,
If you are talking about getting planned and custom dates from multiple projects onto a Calendar, then there's not a perfect way to do this because it only displays planned/actual dates. The only way I can think of including a custom date is to incorporate the custom date field into custom labelling of the calendar items. See attached screenshot which shows how this could be done. If you're not familiar with creating calendars you can have a look at the support documentation.
Instead of doing the above with a calendar, you could create a Task report which includes tasks from the projects you want included. Then, just show the planned completion date field and custom date fields, and then click the Gantt button on the right.....not quite a calendar but it might do the job.
Regards, David