If I understand you right you can have just one on the page instead of having two:
template:
{{day}}
Today is {{day}}
ts:
daysInAWeek: string[] = ['Mo', 'Tu', 'We', 'Th', 'Fr', 'Sa', 'Su'];
toggle = {};
So you can toggle just toggle classes on that element to be fa-plus or fa-minus
You can put (click)="toggle[i] = !toggle[i] on any html element inside your *ngFor temlpate so it will trigger the toggle on click for related element.