"Kami masak ikan di hari Jumat."
Terjemahan:We cook fish on Fridays.
No. You can't say in + name of a day. In is ok for months, but for days you have to use "on". On Monday, etc... in december
The only case where you would use "on" with a month, is for a date.
Ex: On September, the 14th.
The reason is "on" + a day is a precise date, and "On September, the 14th" is also a precise day.