The Predefined Maintenance Windows in Oracle 12c

Oracle documentation states:

"By default there are seven predefined maintenance windows, each one representing a day of the week. The weekend maintenance windows, SATURDAY_WINDOW and SUNDAY_WINDOW, are longer in duration than the weekday maintenance windows. The window group MAINTENANCE_WINDOW_GROUP consists of these seven windows."

Verify by executing:

SELECT WINDOW_NAME,REPEAT_INTERVAL,DURATION,to_char(LAST_START_DATE,'dd.mm.yyyy hh24:mi') "laststart",
    to_char(NEXT_START_DATE,'dd.mm.yyyy hh24:mi') "nextstart",ENABLED "window enabled?",ACTIVE "window open?"
FROM DBA_SCHEDULER_WINDOWS 
WHERE RESOURCE_PLAN='DEFAULT_MAINTENANCE_PLAN';

WINDOW_NAME	REPEAT_INTERVAL	DURATION	laststart	nextstart	window enabled?	window open?
SUNDAY_WINDOW	freq=daily;byday=SUN;byhour=6;byminute=0; bysecond=0	+00 20:00:00.000000	05.02.2017 06:00	12.02.2017 06:00	TRUE	FALSE
SATURDAY_WINDOW	freq=daily;byday=SAT;byhour=6;byminute=0; bysecond=0	+00 20:00:00.000000	04.02.2017 06:00	11.02.2017 06:00	TRUE	FALSE
FRIDAY_WINDOW	freq=daily;byday=FRI;byhour=22;byminute=0; bysecond=0	+00 04:00:00.000000	03.02.2017 22:00	10.02.2017 22:00	TRUE	FALSE
THURSDAY_WINDOW	freq=daily;byday=THU;byhour=22;byminute=0; bysecond=0	+00 04:00:00.000000	02.02.2017 22:00	09.02.2017 22:00	TRUE	FALSE
WEDNESDAY_WINDOW	freq=daily;byday=WED;byhour=22;byminute=0; bysecond=0	+00 04:00:00.000000	01.02.2017 22:00	08.02.2017 22:00	TRUE	FALSE
TUESDAY_WINDOW	freq=daily;byday=TUE;byhour=22;byminute=0; bysecond=0	+00 04:00:00.000000	07.02.2017 22:00	14.02.2017 22:00	TRUE	FALSE
MONDAY_WINDOW	freq=daily;byday=MON;byhour=22;byminute=0; bysecond=0	+00 04:00:00.000000	06.02.2017 22:00	13.02.2017 22:00	TRUE	FALSE

The weekend Windows starts at 6 in the morning and run for 20 hours.
The weekday Windows starts at 22:00 and runs until 02:00 in the morning.

An overview of some important data Dictionary views can be found here