Public Class Form1

    Dim iMonth As Integer
    Dim query As String = " ... AND (TestaFattura.Data = MONTH(" + iMonth + ")) AND (TestaFattura.Data = YEAR(CURRENT_TIMESTAMP)) AND ..."

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        For i As Integer = 1 To 12
            Dim myMonth As String = Microsoft.VisualBasic.DateAndTime.MonthName(i)
            Me.ListBox1.Items.Add(myMonth.ToUpper())
        Next
    End Sub

    Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
        iMonth = Me.ListBox1.SelectedIndex + 1
    End Sub
End Class