If you are an OFW, your SSS contribution payment deadline is more flexible than the deadlines for Local Employees, Self-Employed, Voluntary, Seamen and Household Helpers.

Why are your deadlines more flexible?
It's because you are in another country, and it's not easy looking for a remittance company that accepts SSS payments.
You also need to pay remittance fees to send your payments, so with the flexible deadlines, you can send your SSS payments once or twice a year to save on remittance fees.

What is the SSS payment deadline for OFWs every year?

The last business day of December.

This means you can pay your contributions for January, February, March, April, May, June, July, August, September, October, November or December on any day of the current year. You can pay monthly or quarterly, or for 6 months or for 12 months.

For example, this month is already September. You can still pay for the months from January to June, or from January to September, or from January to December of the current year.

For the last quarter of the year, you are also given more flexibility. You can pay for the last quarter (October November December) on any day during the following month of January.

Sources: 
Circular No. 32-P
Issued November 15, 2006
Subject: New Payment Deadline for Contributions of OFW-Members

Circular No. 2011-003
Issued May 16, 2011
Subject: Revised Payment Deadlines for Contributions and Member Loans
              of All Self-Employed and Voluntary Members and Employers
5.  The payment schedule for OFW-members shall remain as stated under Circular 32-P dated 15 November 2006. 

SSS Payment Deadlines for Voluntary and Self-Employed Members