Now let's see how easy it is to migrate all your appointments, anniversaries, birthdays, etc from Yahoo Calendar to Google Calendar.
Remember that the following approach also works if you plan to move your Microsoft Outlook appointments to Google Calendar.
Just to play safe, do a dummy import first by creating a different calendar for importing external calendars from Microsoft Outlook or Yahoo Calendar.
Step 1. Select the Options link in your Yahoo! Calendar. In the Import and Export section, under subsection "Export to Outlook", Click on the Export Now button.
Step 2. When prompted, specify a convenient name and location for the .CSV file in the "Save As" dialog box.
Step 3. Open Google Calendar Settings and choose Import Calendar. Select the file you created in Step 2 and click Import.
The exported file doesn't support recurring Yahoo! events.
Source: Yahoo Calendar Help | Google Calendar Support | Outlook Help