Adverbs of frequency
1) always come BEFORE the verb, if it consists of one word only.(e.g. goes/fell/cooked, etc)
I always watch cartoons.
David never wore his cap to school.
2) come AFTER to be
I am always,often,usually,never late
but go before "have to"
I always have to tell her..
3) if the verb consists of two or more words(like can remember/doesn't eat,will be canceled)
the adverb usually goes after the first verb/auxiliary(can,does,etc)
Are you definitely going away next weekend?
Clare doesn't often eat meat.
BUT: adverbs of frequency usually go before a negative
so,your sentence
I usually don't have to work on Saturdays.
hope this didn't sound too confusing