Generally speaking adverbs of frequency come before the main verb except the main verb "to be":
I sometimes visit my uncle.
We have often seen him pass by the house.
They are seldom at work.
Occasionally, sometimes, often, frequently and usually can also go at the beginning or end of a sentence:
Sometimes they visit him.
I miss him occasionally.
Rarely and seldom can also go at the end of a sentence (often with "very"):
We see them rarely.
John eats meat very seldom.