Configuring maintenance window for PostgreSQL database in AWS

You can enable a maintenance window for an Amazon RDS for PostgreSQL database instance by using one of the following methods:

Using the AWS Management Console

Open the Amazon RDS console.
In the left navigation pane, navigate to Databases.
Select the database instance for which you want to enable a maintenance window and click Modify.
Under the Maintenance window section, select a day and time for the maintenance window.
Select Continue and then Modify the database instance.

Using the AWS RDS CLI

Open the Amazon RDS CLI.
Use the following command to enable a maintenance window for a database instance:
aws rds modify-db-instance --db-instance-identifier mydbinstance --preferred-maintenance-window Mon:06:00-Mon:09:00

Using the RDS API

Use the ModifyDBInstance action and set the PreferredMaintenanceWindow field in the request parameter.
Send the request to the RDS API endpoint.
You should be able to set the maintenance window to a specific day and time of your choice, provided is at least 30 minutes in the future and not within the next 24 hours. The maintenance window is specified in the ddd:hh24:mi-ddd:hh24:mi format, where:
ddd is the three-letter abbreviation for the day of the week
hh24 is the hour in 24-hour format
mi is the minute

For more information refer to the "Maintaining a DB instance" section of the Amazon RD User Guide.