Individual months in WP Query

It’s possible to pass multiple sets of arguments to date_query, so if you set relation to OR for them, you should get posts that match either, meaning that you could get the posts you want like this:

'date_query' => [
    'relation' => 'OR',
    [
        'month' => 1,
        'year'  => 2019,
    ],
    [
        'month' => 5,
        'year'  => 2019,
    ],
],