Query mysql does not work with variable - PHP

Grouping the answers that are in the comment section, @Jader and @ charlie-pope credits:

$("#agendanome").html() may be sending HTML tags or blank spaces / lines together. To solve use trim () and a strip_tags ():

$agenda = strip_tags(trim($_POST['agenda']));

Sending through Jquery gets better if only the content of the field, rather than the HTML together, uses $("#agendanome").text() .

$.post('calendario.php', {
            mes: mes,
            ano: ano,
            agenda: $("#agendanome").text() // esta linha foi alterada em relação ao código da pergunta
        }, function(resposta) {
                $("#calendario").html(resposta);
        }, 'html');

PHP

 $agenda=urldecode($_POST['agenda']);
    $query = mysql_query("SELECT * FROM 'compromiso' WHERE login LIKE 'alanps' AND agenda LIKE '$agenda' AND ano LIKE '$ano' AND mes LIKE '$mes' AND dia LIKE '$list_day'") or die(mysql_error());

urldecode Is the specific function to remove html spaces from url.

Jquery of the post:

$.post('calendario.php', {
        mes: mes,
        ano: ano,
        agenda: $("#agendanome").html()
    }, function(resposta) {
            $("#calendario").html(resposta);
    }, 'html');

strip_tags It is a function to remove HTML from strings, it can also be used.

[espaço] in URL , it inserts %20 instead of spaces.