Imagine you have during the year couple of situations where the system/Product you provided to client is not available for use, i.e. client is not able to use it.
This is having impact on the clients satisfaction with the product and on any further business development with your client.
Make sure the plan is defined for any maintenance, upgrades and improvements. Make sure you differentiate between mandatory (e.g. technical upgrades and off time) and optional (any improvements requested by client or based on agreement between you and client). If client will agree in advance the frustration will not be so high and even the understanding and cooperation will be much better. Also make sure from technical perspective you minimize the off times.