If you are using HTC Sense 2.5 on your HD2 or in general, you might have noticed that the Calendar Tab’s Agenda View is limited by default to a maximum number of entries matching the next seven days.

You can change that by installing a mod which will automatically replace your original Manila files and will make the Agenda View display the appointments for the next 28 days.

Install the CAB according to your Sense version (check at Settings Tab > About Phone > Software Information > Manila Version) followed by a Soft Reset and you’re set.

If your next 28 days’ appointments are plenty and your device becomes slow while loading them all, you can edit the Registry in order to change the number of days to display appointments for at HKCUSoftwareHTCManilaTweaks.CalendarAgendaShowDays = 28 (change the number to the new number of days you’d like to display). The mod also features some additional registry entries which you can use to tweak it even more.

(Source: XDA Developers)