I maintain our company calendar, which we use to track holidays, employee birthdays, annual meetings, and other events. When I use an Outlook meeting request to invite attendees, why do some users see a 1-day event as spanning 2 days, and how can I prevent this behavior?

Outlook isn't good at handling all-day events for companies whose offices span multiple time zones because the software always stores time-zone information, even for all-day appointments. One possible solution is to use iCalendar or vCalendar items, which ignore time-zone information for all-day events. Instead of sending a meeting request, select the event in the calendar; choose Actions, Forward as iCalendar (or Forward as vCalendar); then, add recipients to the resulting email message. You might want to include a note to let users know that they can double-click the attached .ics or .vcs file to open it and add it to their personal Calendar folders. Note that you can use this technique to select and send multiple events.