A new version (2.5.4) of SharePoint Planner webpart is available to download with the following minor changes :-

• The Mark Weekends feature will now pick up your non working days and hours from SharePoints Regional Settings (WSS 3.0 / MOSS 2007 only)
• The This Week period settings will pick up the start of the week from regional settings

Regional Setting in WSS3.0

You can simply download and install the new version – there is no need to remove the old version first.

Thanks to Michael C and Michael S for these suggestions!