Each reservation must be booked for one or more consecutive nights. To book non-consecutive calendar dates, you must book multiple reservations.
For example: if you would like to stay at a listing for 3 nights, leave for 2 nights, then stay another 3 nights, you must either (1) book two reservations or (2) book one reservation including the nights for which you will not be there. There is no system workaround. This is because a host must take time to prepare and clean the space before and after use for every check-in and checkout.
Feel free to reach out to your host in your message thread to discuss booking options. If your host agrees, he or she can make a Special Offer that accounts for the nights you need at a discounted price to exclude the ones you do not. However, this will keep the host's calendar blocked, therefore, he or she cannot book a reservation during the nights you will not be using the space.