I'm having the same problem and also the 'Previous Month' button doesn't appear to work on the live site either. I think it is a bug to do with today's date as I have tried changing my system calendar forward a couple of days and all is working again. I am guessing some calculation to do with the end/beginning of the month isn't right. Have spent 3 hours hitting my head against the wall today on this....

I've also spent this afternoon on this issue. I'm using Date.setUTCMonth() in Common.js to find the first day of a month but it's always setting +1 month. I would call that a bug of the browser (it was working well so far) but it's behaving the same way in both IE7 and FF2. I also notices some strange timezone shifts (which seem to have something to do with the summer time).

I've switched to Date.setUTCFullYear(year, month, day) and it works fine now. This fix will be included in the next release but feel free to ask me for a fresh build if you need it now (support@daypilot.org).