Doubt in extending the Payperiods

Hi Experts,

As a yearend activity I am trying to extending the payperiods upto Dec 2017 for a US client.

I joined the support project very recently and got confusion in generating the payroll periods.

The payroll is bi-weekly payroll.  I got confusion in Paydayrule , Number of days and Determine period number.

At present the last record is ---> payroll year - 2017 ; Payroll period - 01; Start date - 18/12/2016; End date - 31/12/2016; prev P year: 2016 ;

Previous P period : 26 ;

Here I am in confusion in maintaining the values of  Paydayrule , Number of days and Determine period number.

Is there any way to find out which Paydayrule is used previously ?

Also for some years there were 27 payperiods and for some other years there were 26 only. What is the reaosn for the this. What is the best practice.

Regards,

Sairam.