1 Make sure jQuery is present and working on the page in question - I found I had to download from the jQuery site (http://docs.jquery.com/Downloading_jQuery) and use a relative link. Using the CDN hosted jQuery didn't work for me.
<script src="/Shared%20Documents/jquery.js" type="text/javascript"></script>
This link was useful - http://www.threewill.com/2012/01/adding-jquery-to-sharepoint/
2 Add a content editor web part to the page with this code:
<script language="javascript" type="text/javascript">
$(document).ready(function () {
ExecuteOrDelayUntilScriptLoaded(ShowCalendarRibbon, "sp.ribbon.js" );
function ShowCalendarRibbon() {
try {
var elem = document.getElementById("MSOZoneCell_WebPartWPQ2");
if(elem != null) {
var theEvent = new Array()
theEvent["target"] = elem
theEvent["srcElement"] = elem
WpClick(theEvent)
}
}
catch (e) {
alert("Problem initializing ribbon");
};
};
});
</script>
N.B. Add this code through the HTML source of the content editor:

You may need to change the ID for the Calendar depending on the web parts on your page, but mine was - MSOZoneCell_WebPartWPQ2
Set "Chrome Type" to "None" to hide the content editor on the page.
This post provided most of the info - Select the ListView Web Part Automatically on AllItems.aspx
This post also helped - http://ikarstein.wordpress.com/2010/06/15/how-to-activate-sharepoint-ribbon-tab-by-javascript-code/
Using a query string is also an option http://www.sharepointdevelopment.me/2010/11/set-active-ribbon-tab-from-querystring/ - E.g. ?InitialTabId=Ribbon.Calendar.Calendar - but this didn't seem to work for me