When you want to send a one-time email, there's no way to specify what calendars to mix-in with the one you're sending.
The first time you want to specify mix-ins for calendar email, you need to go through the Email Publishing Wizard as if you want to send a regularly scheduled email.
- Display the appropriate calendar and click Email, and then click Set up Scheduled Email.
- Create your distribution list, click Next, and in step 2, under What other calendars' events should we mix-in?, select the calendars you want to mix in.
- Toward the top of that same page, check Send the first email as soon as I finish.
- Complete the wizard steps and click Finish.
- If you don't want to send a regularly scheduled email, open the wizard again and delete the schedule.
The next time you use the wizard to send a one-time email, the same mix-ins you specified for the scheduled email will be included in the one-time email.