Time expression: The adessive case (-l)
1. To indicate the time when something occurs/is occurring. The suffix is (-l) and it literally means "on" or "in", depending on the context.
päeval (in the daytime)
hommikul (in the morning)
õhtul (in the evening)
2. Date and month
Seitsmendal veebruaril (On February 7th)
Estonian nouns are formed differently from Finnish. Most noun stems are based on the genitive case. Unfortunately, the genetive case in Estonia is irregular and has to be memorize by heart when learning new words.
The adessive -l is added directly to the genetive case.
Päev (nom.) -> päeva (gen.) -> päeval (ade.)
Hommik (nom.) -> hommiku (gen.) -> hommikul (ade.)
Õhtu (nom.) -> õhtu (gen.) -> õhtul (ade.)
Your example is correct.