const myObject =  {

      [_selectedDay] :
        {
        toDo:'workout',
        selected: true,
      }
  }

//I expaned [0]
(5) [{…}, {…}, {…}, {…}, {…}] "object"
0:
->2019-06-03: {toDo: "workout", selected: true}
1: {2019-06-04: {…}}
2: {2019-06-05: {…}}
3: {2019-06-06: {…}}
4: {2019-06-07: {…}}

myObject[0].toDo = 'workout'

var firstKey = Object.keys(myObject)[0];
myObject[firstKey].todo = 'workout'

(5) [{…}, {…}, {…}, {…}, {…}] "object"
0:
->2019-06-03: {toDo: "workout", selected: true}
1: {2019-06-04: {…}}
2: {2019-06-05: {…}}
3: {2019-06-06: {…}}
4: {2019-06-07: {…}}