Heartland Retail uses a common week-date standard (ISO) for numbering weeks. All weeks (when running weekly or 'last week' reports) start on Monday and end on Sunday.
According to the ISO week date standard as an example:
2015 there were 53 weeks.
Following the Monday to Sunday week format, week one in 2015 was Monday, 12/29/2014 to Sunday, 1/3/2015 and week 53 in 2015 was Monday, 12/28/2015 to Sunday, 1/3/2016.
This means that week one in 2016 is Monday, 1/4/2016 to Sunday 1/10/2016. Therefore the week one comparison of 2015 to 2016 is the week starting 12/29/2014 to the week starting 1/4/2016.