I want to display the days and date also by the relationship between the coach and working times

Here a want to add Appointments available for the coach by using two tables…one for date_days the second for trainer and I make a 1-m relation between two tables.

Dateday Table...

class Dateday extends Model
{

    public function trainer()
    {
        return $this->belongsTo(Trainer::class, 'trainer_id', 'id');
    }
}

 Trainer Table...

class Trainer extends Model
{
    public function datedays()
    {
        return $this->hasMany(Dateday::class);
    }
}

Trainer index ...

Show this way you give me an error, And I want To add a day and date and start_date/end_date to the index page for trainer Who can do this??

 @foreach ($trainers as $trainer)
      <tr>
        <td>{{ $trainer->id }}</td>
        <td>{{ $trainer->firstname }}</a></td>    
        <td>{{ $trainer->lastname }}</a></td>    
        <td>{{ $trainer->phone }}</a></td>    
        <-- Show this way you give me ero -->
        <td>{{ $trainer->dateday->day}}</a></td>  
        <td>{{ $trainer->dateday->date}}</a></td>  
      </tr>
@endforeach

Answer

In your Trainer class, you have defined a function datedays, but in your template you are using $trainer->dateday without the s.

Regarding your edit $trainer->datedays is a Collection of all the datedays associated with the trainer (because of the ->hasMany), so you cannot get the property of an individual item. If there should only be one date, the relation should be changed (->hasOne), or if there should be more dates possible, you should think about how you want to show that. If you (for example) only want to show the first date you could use {{ $trainer->datedays->first()->day }} or you could loop over all dates using @foreach ($trainer->datedays as $dateday) and use {{ $dateday->day }} in the loop.