To-dos with a deadline will automatically appear in your Cupla calendar on their due date. To enable this, make sure your to-do has a deadline set.
Adding a deadline to a to-do
Open the to-do (or tap + to create a new one).
Toggle Deadline on.
Set the Due by date and time.
Tap the Save button in the top right-hand corner.
The to-do will now appear in your Cupla calendar on its due date. Repeating to-dos will show all occurrences in the calendar.
See also: How do I create a to-do that repeats?