Editing a text file from plugin menu

First of all put these all code inside in init hook, and use plugin_dir_path() func to get plugin dir path.

add_action('init',function(){

    $plugindir = plugin_dir_path( __FILE__ );

    if(isset($_POST['addition'])){ 

        $file_open = fopen($plugindir."dates.txt","w+");
        fwrite($file_open, $_POST['addition']);
        fclose($file_open);
    }
        echo '<form action="'.$_SERVER['PHP_SELF'].'" method="POST">
        <textarea name="addition" COLS=50 ROWS=20>';

        $datalines = file ($plugindir."dates.txt");

        foreach ($datalines as $zz) {
            echo $zz;
        }                   
        echo '</textarea><input type="submit" name="button" value="Save Evenings Before Solemnities File"></form>';
        echo '</td>';
        exit;

});

Try this above code.