Both Google and Yahoo! have popular calendar services that integrate with their suite of web applications, like their popular e-mail or news services. Because people use multiple e-mail accounts, often they will have multiple calendars as well. By combining these calendars you can better organise yourself and keep track of events coming in from both services. Since each calendar can produce and read an iCal-compliant version of your calendar, you can easily sync the two services by pointing one at the other's shared iCal URL.
Convert your Yahoo! Calendar to Yahoo! Calendar Beta. (See Resources) Open Google Calendar and create a new calendar.
Open your new Google calendar's settings. Click the green iCal box under the "Private Settings" section and copy the provided URL.
Open your Yahoo! calendar and add a new calendar. Choose to subscribe to a calendar. Paste the iCal URL from Google into the provided text-entry field and click "Next."
Choose to share your calendar on your Yahoo! calendar and enter your e-mail address. Click "Save" to have an e-mail sent to your address that will contain the Yahoo! iCal URL.
Copy the Yahoo! iCal URL from your e-mail and open up your Google calendar. Click "Add" under the "Other Calendars" section and choose the "Add by URL" option. Paste the Yahoo! iCal URL into the text-field pop-up and click "Add" to finish syncing your calendars.