Dennis,
The spring flowering primroses are primulas. They are a different
species than oenethera - evening primroses, which, by and large bloom in
summer. Most primulas are native to Europe and Asia - most oenetheras are
native to the Americas. Primulas come in a larger variety of colors. (Red,
pink, purplish blue, yellow, white, orange). Oenetheras come mostly in
different shades of yellow, although one variety , oenethera speciosus is
pink and sometimes white.
A good many evening primroses only open their flowers at night, because
they are pollinated by moths. However, there are quite a few exceptions.
Primulas are all open during the day.
Hope this helps you.