Vue JS CheckBoxGroup with a prop array as v-model

I am stuck at making a CheckBoxGroup with a prop array as v-model. I have read the vuejs guide: https://vuejs.org/v2/guide/forms.html#Checkbox which has the v-model array in the data of the same component, but it is obviously pretty useless if I want to make this component reusable and insert the v-model via props and for example check some of the boxes from "outside". So I tried following:

CheckBoxgroup.vue

<template>
  <div>
    <label v-for="day in allDays" :key="day">
      <input v-model="checkedDays" type="checkbox" :value="day" />
      <span></span>
    </label>
    <div>Checked days: </div>
 </div>
</template>
<script lang="ts">
import Vue from 'vue'
import { Component, Prop } from 'vue-property-decorator'

@Component
export default class CheckBoxGroup extends Vue {
  @Prop() checkedDays!: string[]

  @Prop() allDays!: string[]
}
</script>

Index.vue

<template>
  <div>
    <checkbox-group :checked-days="checkedDays" :all-days="allDays" />
  </div>
</template>

<script lang="ts">
import { Vue, Component } from 'vue-property-decorator'
import CheckboxGroup from './checkBoxGroup.vue'

@Component({
  components: { CheckboxGroup },
})
export default class Index extends Vue {

  // This list would usually come from an API
  allDays = ['Monday', 'Tuesday', 'Wednesday']

  checkedDays = ['Monday']
}
</script>

So the code is working almost fine, but I am getting

[Vue warn]: Avoid mutating a prop directly since the value will be overwritten whenever the parent component re-renders...

Is there any way around it? Any help would be appriciated.