Just pass selectedDay to methods. For removeAbsence it is easier to pass index instead of item itself.

addAbsence(day: DayOfWeek, absence: ITimesheetAbsence): void {
  day.absences.push(absence);
}

removeAbsence(day: DayOfWeek, absenceIndex: number): void {
  day.absences.splice(absenceIndex, 1);
}