A calendar widget is a must have for the N900 and almost everyone has it on their homescreen. Unfortunately it is not customisable and a tap on it opens the day’s agenda and not the month view, something I hate. Fortunately nicolai has produced a replacement for it that is based on the default N900 theme and looks just like the default, but with a few additions and the ability to customise. Get it from extras-devel, search for ‘Calendar Home Widget’.
You can now set the duration for which the calendar checks, the number of rows the widget shows (2-9), the calendars it displays, and what mode it opens when tapped on. Here is what it looks like in the ‘Large’ size, notice the default widget for comparison sake. The extreme right of the replacement widget shows whether an entry is a birthday or has an alarm set, it also shows the month on top.
The only disadvantage is that it does not adapt to the current theme like the inbuilt widget. Hopefully, a future update will fix that as well. Follow this TMO thread for the latest.