Allow users to set preferences for little drop-down calendars (ie. to select date of an invoice or a sale) so week begins on Monday.
When you create a new invoice or transaction, you select the date and a little calendar appears. This calendar displays with the week beginning on Sunday – however, all my other calendar apps begin the week on Monday, and so this is incongruous. An option in 'Settings' to change which day the week starts on would surely not be difficult to implement, and would allow users to choose this for themselves.