According to wikipedia,the last week of the ISO week-numbering year is the week before week 01; in accordance with the symmetry of the definition, equivalent definitions are:

* the week with the year’s last Thursday in it
* the week ending with the Sunday which is nearest in time to 31 December
* the week with December 28 in it
* the last week with the majority (four or more) of its days in the ending year
* the week starting with the Monday in the period 22 – 28 December
* the week with the Thursday in the period 25 – 31 December
* the week ending with the Sunday in the period 28 December – 3 January
* If 31 December is on a Monday, Tuesday, or Wednesday, it is in week 01 of the next year, otherwise in week 52 or 53.

So this plugin when calculating the weekno and if there is a WKST specification in the rule, uses “the week with December 28 in it” to calculate the last week and handle negative BYWEEKNO’s that way.

The logic is something like this

• get the end of the year (ie 28th Dec)
• determine the weekno (will be 52 or 53?)
• go to the start of that week, using wkst
• “save” the this and next 6 dates as our week