Need Workday formula explained

The Workday function works thus:-

"=WORKDAY( start_date, days, [holidays] )"

where the arguments are as follows:

start_date - The initial date, from which to count the number of workdays.
days - The number of workdays to add onto start_date.
[holidays] - An optional argument, which specifies an array of dates (in addition to weekends) that are not to be counted as working days.

Your formula is

Formula:

=WORKDAY(DATE(2016,1,3)+((ROW()-1)*7),1,Holidays!$A$1:$A$26)

In Cell A1 you have this formula:-

Formula:

=DATE(2016,1,3)

Which is your reference Date Sunday 3rd Jan and is copied unchanged to all the cells in column A and is therefore important.

Formula:

=+((ROW()-1)*7)

Adds 7 to your reference date for every row you go down column A

Formula:

Holidays!$A$1:$A$26

Is your list of holidays that the Workday function will ignore.

So back to your Formula

Formula:

=WORKDAY(DATE(2016,1,3)+((ROW()-1)*7),1,Holidays!$A$1:$A$26)

This means

Using Sunday 3rd Jan as a reference, adding 7 days each row, and adding one day. [ So Looking at Each Monday from 4th Jan 2016 ]
And ignoring My Holidays Listed in

Formula:

Holidays!$A$1:$A$26

Return the First Workday.