Nairobi National Park has specific operating hours for terrestrial/Marine Parks and Nairobi Orphanage but the national parks and reserves are open everyday of the week.
Below is a message from KWS about the operating hours:
“KWS Parks and Reserves are open everyday of the week.
Terrestrial/Marine Parks: 6.00 a.m. – 6.00 p.m
Nairobi Orphanage & Nairobi Safari Walk: 9.00 a.m. – 5.30 p.m”
If you want to visit the park, you can go there any time of the day. Evening or morning hours are the best times to visit the park.