I need to group per week based on the registration date ?
It's not quite clear what CalendarWeekRule do you have in mind, so assuming you want the simplest (FirstDay), i.e. the formula like
Week = 1 + (DateTime.DayOfYear - 1) / 7
EF Core supports translation of the DayOfWeek to SQL, so you could use something like this (not sure if you need Year):
.GroupBy(g => new { g.Trsdt.Year, Week = 1 + (g.Trsdt.DayOfYear - 1) / 7 })
using System.Globalization;
.GroupBy(
g => CultureInfo.CurrentCulture.Calendar.GetWeekOfYear( g.Trsdt,
CalendarWeekRule.FirstDay,DayOfWeek.Monday )
)