On my Wife's LG GT540, the calendar has 2x 3rd of April, as you can see in the following picture:
As funny as it is, it isn't very helpful...
I'm wondering if anyone else has heard of this and/or knows a solution?
If your phone is still running Donut, did you know that an Eclair (Android 2.1) update should be available for that phone, Eclair starting rolling out to the Middle East in September and the rest of the world should have had it by the end of 2010. See the official GT540 update guide on LG's website. You should be able to download the necessary LG Mobile Support Tool software from here.
I would hope that such a glaring bug has been fixed in the updated calendar/widget.