In SharePoint 2010, the calendar views are rendered via a script on the client side. This is a significant improvement over the heavy calendars of the previous versions (2003 and 2007), and in the 2013 version client side rendering has been extended to all list views.
Unfortunately, the SP 2010 calendar also came with a bug that I reported a couple years ago. It is made of two layers – one for the grid and one for the events – and when the calendar gets resized the two layers get out of sync. Really annoying! As far as I know, Microsoft has never fixed this. I keep receiving reports of this bug from users, especially as it occurs when calendar views are combined with the Easy Tabs.
If you have to deal with this annoying behavior, here is a simple workaround: go to the Web Part settings and give the Web Part a fixed width.
Hope this helps!
Path to SharePoint... and Beyond! 
Thanks Christophe, Is there any alternate way to display sharepoint 2010 tabbed calendars, I used easy tabs for sharepoint 2007 calendars and it worked great, but now we migrated to 2010 and I see the events come and stick in the top left corner of the calendar. I really appreciate your response.
Are you saying that the solution described in this post doesn’t work for you?
Thanks for your reply , Yes, it is not working in 2010 for tabbed calendars. Did you release any updated easy tabs version that fixes tabbed calendars.If not can you please suggest any alternate way to display calendars in tabbed. Most of my pages have tabbed calendars in 2007 and now I need to do a visual upgrade and noticed events come and stick in the top left corner of the calendar.
I really appreciate your response.
Christophe, Did you get a chance to look at my previous commnet. I am just stuck
Sorry, the only workaround I know is the one described in this post. I don’t know why it doesn’t work in your case.