I am trying to set custom validity for a date field, that throws an error message when I select any day which is not Sunday in Lightning Web Component

Question

This is my lightning input for date, which has a onchange event dateHandleChange. This is my first lwc. te.html

<div class=" slds-size_1-of-4 ">
  <lightning-input type="date" label="Pick a Sunday" name="date" value={dateEntry} onchange={dateHandleChange} required></lightning-input>
</div>

This is the js file which handles the onchange event, please can someone help. te.js

    dateHandleChange(event){
        let errorMessage = 'Please enter a Sunday.';
        let fieldSel = this.template.querySelector(".date");
        this.dateEntry = event.target.value;
        let val = this.dateEntry.getDate();

        if(val == '0'){
            fieldSel.setCustomValidity('');
        } else {
            fieldSel.setCustomValidity(errorMessage);
        }

        fieldSel.reportValidity();
    }