Watch LifeHacker's tutorial on how to migrate your Google Calendar into a different account:
There are two ways to migrate your Google Calendar. One of them takes a bit longer, but we'll only use that method for your main calendar (the one that, by default, is named after your Gmail address). All of the sub-calendars, or "delegates", of the account are easier to migrate, so we'll do that second.
To migrate your main calendar, go into your Google Calendar settings and click on the Calendars tab. You'll see an option to export all your calendars, at once, to your hard drive in iCalendar format. Export them to your drive and unzip them if necessary.
Next, go to your new account (it's easiest to have each account logged into Google in a separate browser), go to Settings > Calendar Settings > Calendars and create a new one. Name it whatever you like, click Create Calendar, and then hit the import button at the bottom of the page. Choose the
.ical file corresponding to your main calendar and upload it, choosing the empty Google Calendar you just created as its destination. You'll see that it shows up in calendar view.
If you have more calendars you want to import beyond the default, we're just going to share them with the other account. Go back to the Calendars tab of Account 1's settings and hit the "Share This Calendar" button next to a calendar. Type in the Account 2's address and hit Enter. Change the permissions in the drop-down window to "Make Changes AND Manage Sharing" and hit Save. Account 2 will receive an email saying that you've shared a calendar, and provide a confirmation link. The first time you receive an invitation, you'll need to click on this link, but afterward the calendar will just show up automatically. You can then go back to the Calendars tab of your new account's settings, hit the "Shared: Edit Settings" link and remove Account 1 at the bottom, by clicking the trash can icon. (This removes Account 1 from sharing the calendar, so Account 2 is left with full ownership of the calendar.) Repeat this for each of your calendars and you'll have migrated everything you need. This method works for both regular Google accounts as well as Google Apps.