> I want to change the week start from Sunday to Monday. How can this be
I don't think you can change that in the calendar plugin that is shipped
with SquirrelMail, unless hacking the source. Try downloading Shared
Calendars <http://www.squirrelmail.org/plugin_view.php?id=105>, install it
according to the instructions (replacing the default one) and you will be
able to configure which day you want the week to begin with.