Grouping by Week Across Years

GerryL , Assume you have a term start date

//All these column are in a separate date table marked as date table

then you can have columns like

Day of period  = datediff([term Start Date],[date], day) +1

Week of period  = quotient(datediff([term Start Date],[date], day) ,7) +1 //Based on start Date

or

Week of period  =

var _1 = [term Start Date] - weekday([term Start Date],2)+1  //Monday of term start , use 1 for Sunday in weekday

return

quotient(datediff(_1,[date], day) ,7) +1 //Based on start Date

Create rank on term start date

Period Rank = RANKX(all(Period),Period[Term Start Date ],,ASC,Dense)

This Period = CALCULATE(sum('Table'[Qty]), FILTER(ALL(Period),Period[Period Rank]=max(Period[Period Rank])))
Last Period = CALCULATE(sum('Table'[Qty]), FILTER(ALL(Period),Period[Period Rank]=max(Period[Period Rank])-1))

Period Day = CALCULATE(sum('Table'[Qty]), FILTER(ALL('Date'),'Date'[Period Rank]=max('Date'[Period Rank]) && [Period Day] =max([Period Day])))

Last period Day  = CALCULATE(sum('Table'[Qty]), FILTER(ALL('Date'),'Date'[Period Rank]=max('Date'[Period Rank])-1 && [Period Day] =max([Period Day])))

Period Week= CALCULATE(sum('Table'[Qty]), FILTER(ALL('Date'),'Date'[Period Rank]=max('Date'[Period Rank]) && [Week of period] =max([Week of period ])))

Last period Week= CALCULATE(sum('Table'[Qty]), FILTER(ALL('Date'),'Date'[Period Rank]=max('Date'[Period Rank])-1 && [Week of period] =max([Week of period ])))

This video can help to understand

Time Intelligence, DATESMTD, DATESQTD, DATESYTD, Week On Week, Week Till Date, Custom Period on Period,
Custom Period till date: https://youtu.be/aU2aKbnHuWs&t=145s

DAX Calendar - Standard Calendar, Non-Standard Calendar, 4-4-4 Calendar
https://www.youtube.com/watch?v=IsfCMzjKTQ0&t=145s