When and Where To File .....

In general, Form 3520 is due on the date your income tax return is due, including extensions.

From the IRS form 3520 instructions ...   https://www.irs.gov/pub/irs-pdf/i3520.pdf

You will ALSO need to file FBAR by April 15th.
<a rel="nofollow" target="_blank" href="http://bsaefiling.fincen.treas.gov/NoRegFBARFiler.html">http://bsaefiling.fincen.treas.gov/NoRegFBAR...>

As correctly stated above, if you are the owner of or have signature authority over funds > 100K in a foreign bank (foreign financial institution, FFI) you need to file the following with your 2016 tax return:

1) FinCEN Form 114, Report of Foreign Bank and Financial Accounts
2) IRS Form 3520, Annual Return To Report Transactions With. Foreign Trusts and Receipt of Certain Foreign Gifts.

You also need to complete:

3) IRS Form 1040, Schedule B, Part III
4) IRS Form 8938, Statement of Specified Foreign Financial Assets with your income tax return.

<a rel="nofollow" target="_blank" href="https://www.irs.gov/pub/irs-pdf/i8938.pdf">https://www.irs.gov/pub/irs-pdf/i8938.pdf</a>
<a rel="nofollow" target="_blank" href="https://www.irs.gov/businesses/comparison-of-form-8938-and-fbar-requirements">https://www.irs.gov/bu...>

Consult an experienced tax practitioner before you file in April especially since failure to file penalties are substantial.

Clarification - FinCEN Form 114 is a not an IRS form. It goes to the Treasury Department. This is noted correctly (with link) in the first comment by TaxGuyBill.