Apparently there is no functionality to change your default calendar in Google Calendar. However there is a workaround. Note that first calendar in the list "My calendars" on the left (you may need to expand it) is the default and it is created when you register in Google Calendar.
Shortly speaking you need to export both of your calendars into local files, erase all events from default calendar and unsubscribe from other calendar that you want to be the default, create new calendar to host old default calendar's events, rename default calendar to other calendar and import events back appropriately.
Here are the step by step instructions. Let's say you have a default calendar "Events" and other calendar called "Daily plan" that you want to make the default. Here is what you need to do:
- Click on the small triangle next to current default calendar (first in the list) and select "Calendar settings"
- Click in "ICAL" button next to bottom-most "Private Address" section. This will save a local file on you computer that contains all the events from your current default calendar. Rename the file form "basic.ics" to something more reasonable, e.g. "dafault.ics"
- Perform the same operation for the other calendar and save it as "other.ics"
- Go to "Calendar settings" from "Settings" menu on the top-right from the main page and select "Calendars" tab
- Click "Delete" button next to default calendar. Note that this does not delete the calendar, but only removes events from it. It is impossible to completely delete default calendar without deleting your Google Account.
- Click "Unsubscribe" button next to the other calendar. This will actually remove the other calendar from your account.
- Click on the default calendar name (e.g. Events) and rename it to other calendar that you wanted to be the default (e.g. Daily plan).
- Return back to the list of calendars by repeating step 4
- Create new calendar that will host your old default calendar events with the same name (e.g. Events). You can create a calendar by clicking on "Create new calendar" button. Afterwards return to the list of calendars again.
- Now click "Import calendar" link next to "Create new calendar" button.
- Select file "default.ics" and import it into newly created calendar "Events". This operation may take longer or shorter time depending on the number of events that you have. Be patient, it took me about 2 minutes to load 300 events.
- Perform the same operation and import "other.ics" into default calendar current named as other one (e.g. Daily plan).
- You may want to change calendars' colors to be like before the change (they must be swapped by now). You can do that from the main page by clicking on the triangle next to calendar name in the list and selecting appropriate color. You can also change additional settings like calendar description from the calendar settings, which can be reached as described in step 1.
- Viola! You are done.
Please keep the exported files before you are sure that everything worked well. In case you have messed something up you can always restore your calendars from these files. Please let me know if something is wrong with these instructions.