Possible redundant code improvement first here is the code:

Part 1
    $('#monhpf').keyup(function()
    {
        /* General variable(s) */
        var monhpf = $('#monhpf').val();

        if(monhpf=="")
        {
            $('#monhpf').css('border-color',red);
            $('.mondayErrors').text(emptyField).show();
        }else       
        if(monhpf.length &gt; 0)
        {
            /* Verifies the charset of the zip code */
            $.post('functions/hoursCharset.php',{hoursPHP:monhpf},function(hoursCharset)
            {
                if(hoursCharset==true)
                {
                    /* Charset valid*/
                    $('#monhpf').css('border-color',green);
                    $('.mondayErrors').hide();
                    mondayErrors = false;
                }else
                {
                    /* Charset not valid */
                    $('#monhpf').css('border-color',orange);
                    $('.mondayErrors').text(invalid).show();
                    mondayErrors = true;
                }
            });
        }
    });
Part 2  
    $('#monmpf').keyup(function()
    {
        /* General variable(s) */
        var monmpf = $('#monmpf').val();

        if(monmpf=="")
        {
            $('#monmpf').css('border-color',red);
            $('.mondayErrors').text(emptyField).show();
        }else       
        if(monmpf.length &gt; 0)
        {
            /* Verifies the charset of the zip code */
            $.post('functions/minutesCharset.php',{minutesPHP:monmpf},function(minutesCharset)
            {
                if(minutesCharset==true)
                {
                    /* Charset valid*/
                    $('#monmpf').css('border-color',green);
                    $('.mondayErrors').hide();
                    mondayErrors = false;
                }else
                {
                    /* Charset not valid */
                    $('#monmpf').css('border-color',orange);
                    $('.mondayErrors').text(invalid).show();
                    mondayErrors = true;
                }
            });
        }
    });


As you can see part 1 and part 2 are pretty much the same except for the #monhpf and #monmpf variables. My question is, how can I improve this code to be smaller? Like in a single function for example.
1)

Take advantage of javascript's truthy and falsey values.

Boolean(undefined); // =&gt; false
Boolean(null); // =&gt; false
Boolean(false); // =&gt; false
Boolean(0); // =&gt; false
Boolean(""); // =&gt; false
Boolean(NaN); // =&gt; false

Boolean(1); // =&gt; true
Boolean([1,2,3]); // =&gt; true
Boolean(function(){}); // =&gt; true


More here: http://james.padolsey.com/javascript/truthy-falsey/

if(monmpf == "") => if(!monmpf). 

if(minutesCharset == true) => if(minutesCharset)

if(monhpf.length &gt; 0) => if( monhpf.length )

2)

You can refer to the current element by using $(this).

$('#monmpf').keyup(function () {
// el refers to $("monmpf")
var el = $(this);


3)

if (monhpf.length &gt; 0) is redundant since you already checked the string value with this, if(monmpf == "").

4)

You're using too many global variables, such as emptyField, invalid, green, red, orange, etc.
Try wrapping the variables inside a meaningful namespace, like 

var COLORS = { red: "red", green: "green" };
var MESSAGE = { invalid: "Wrong", emptyField: "" };


5)

mondayErrors? I think you should rename this variable.

6)

Break up functions that are longer than 8-12 lines of code into smaller ones.

With all that said, here's what I came up with.

var getRequestObject = function (name, value) {
    var obj = {};
    switch (name) {
        case "monhpf":
            obj.url = 'functions/hoursCharset.php';
            obj.paramObj = {
                hoursPHP : value
            };
            break;
        case "monmpf":
            obj.url = 'functions/minutesCharset.php';
            obj.paramObj = {
                minutesPHP : value
            };
            break;
    }
    return obj;
};
var getPostFunc = function ($el) {
    return function (isCharset) {
        mondayErrors = !isCharset;
        if (isCharset) {
            $el.css('border-color', green);
            $('.mondayErrors').hide();
        } else {
            $el.css('border-color', orange);
            $('.mondayErrors').text(invalid).show();
        }
    };
};
$('#monhpf, #monmpf').keyup(function () {
    var $el = $(this),
        request,
        value = $el.val(),
        id = $el.attr('id');

    if (value) {
        request = getRequestObject(id, value);
        $.post(request.url, request.paramObj, getPostFunc($el));
    } else {
        $el.css('border-color', red);
        $('.mondayErrors').text(emptyField).show();
    }
});