WPML custom string translation get outside of container

I am trying to add a custom string to translate in wpml, but when I use the _e(…) the content jumps outside of container

if I use like this:

echo '<h1>' . $month . ' ' . $day . '</h1>';

the html is ok:

<h1>September 29</h1>

but if I use with translation like this:

echo '<h1>' . _e($month, 'simultan') . ' ' . $day . '</h1>';

the translated string jumps out of h1:

Septembrie<h1>29</h1>