[jQuery] $(".my_input").focus() shouldnt work?

[jQuery] $(".my_input").focus() shouldnt work?

hey guys, could u help me?
this is my lines:
$("#month").keypress( function(){
        len = $(this).val().length;
        if(len == 1){ document.getElementById("day").focus(); }
    });
this firebug give me an error.
if(len == 1){ $("#day").focus(); }
this do nothing...
what am i doing wrong
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