Displaying More Than One Message On Alert Box

I am in the process of learning javascript and I've been looking at this code for the longest time ever. So far I got most of it done. For example I leave all the required fields empty, it gives me the alert message of the first field and not the alert messages of all the others. What I want to know is how can you go through and check the whole form first before submitting and then when there is any error on any field, shows just one alert message containing all the error messages.

Code for the form
<form action="mailto:hummdedum@felloff.com" method="post" name="form" onSubmit="return isFormValid();">
* First Name: <input type="text" name="FName" id="FName" onblur="checkFName();"/><label id="labelFName"></label><br />
* Last Name: <input type="text" name="LName" id="LName"onblur="checkLName();"/><label id="labelLName"></label><br />

* Password: <input type="password" id="pw" name="Password" onblur="checkpw();" id="pw"/><label id="labelpw"></label><br />
*Re-type Password: <input type="password" name="2Password" id="pw2" onblur="checkpw2();" /><label id="labelpw2"></label><br />

I am a: <br />
<input type="radio" name="status" value="fresh" /> Freshman<br />
<input type="radio" name="status" value="soph" /> Sophomore<br />
<input type="radio" name="status" value="jr" /> Junior<br />
<input type="radio" name="status" value="sr" /> Senior<br />

I am taking classes in the: <br />
<input type="checkbox" name="semester" value="fall" /> fall time<br />
<input type="checkbox" name="semester" value="spring" /> Spring time <br />

My favorite element is:
<select name="element" id="element">
<option value="">select one</option>
<option value="fire">Fire</option>
<option value="earth">Earth</option>
<option value="water">Water</option>
<option value="air">Air</option>
</select><br />

*Birthday: <input type="text" id="BDay" name="Birthday" onblur="checkBDay();"/><label id="labelBDay"></label><br />
*E-Mail: <input type="text" id="email" name="Email" onblur="checkEmail();"/><label id="labelEmail"></label><br />
<input type="submit" value="Submit" />
<input type="reset" value="Clear" />

I want to also validate birthday.. and I tried using regular expression with leap years but the expression is too hard for me to think of. So I am gonna try using split() but I don't know and for the clear button. Since I blur functions, how would I just clear all the blur statements = like a restart of the form and then when the user enters the field the blur function still works?