I often forget today's date. This tiddler inserts an empty Google Calendar into the sidebar of my wiki so that I can quickly glance at it.
Just create a new tiddler, call it "Calendar" (or whatever you want) and paste this embed code into the body of your tiddler:
<iframe src="https://www.google.com/calendar/embed?src=qqndj42ted2fn9ph0v6bc0ij34%40group.calendar.google.com&ctz=America/New_York" style="border: 0" width="800" height="600" frameborder="0" scrolling="no"></iframe>
Then -- and this is the important part -- add or create this tag:
$:/tags/SideBar. Then save the tiddler and you ought to see a Calendar tab in your sidebar.
You can of course use any of your current Google Calendars -- just go to the Calendar Details tab, scroll down to Embed This Calendar, and grab the embed code.