WPML custom string translation get outside of container

When you use _e your echoing the translated string. But you already have an echo so just use __(.

This is the correct code for your example

echo '<h1>' . __('September', 'simultan') . ' ' . $day . '</h1>';

If you want to add variables in your translations you should use sprintf like this:

$date = sprintf ( __('%s %d', 'simultan'), $month, $day );
echo '<h1>' . $date . '</h1>';

You might want to also have a look at date_i18n function since you are translating dates.

Reference:

WordPress I18N

sprintf manual