This is the 288th most frequent Latvian word.
septembrī
"In September."
The word 'septembrī' here indicates the time when the action (starting the new school year) takes place. It is used in the locative case to denote time.
Septembrī sāksies jaunais skolas gads.
In September, the new school year will start.
In this sentence, 'septembrī' specifies the month during which the event (birthday) occurs. Again, it employs the locative case for indicating time.
Manai vecmāmiņai dzimšanas diena ir septembrī.
My grandmother's birthday is in September.
Here, 'septembrī' expresses the temporal frame (September) associated with the activity (harvesting crops). It is similarly in the locative case to reflect time.