<%@ Page Language="C#" AutoEventWireup="true" CodeBehind="form.aspx.cs" Inherits="project.form" Culture="es-SP" UICulture="es-SP" %>

<asp:TextBox ID="calendar" runat="server" CssClass="form-control" placeholder="01/01/2000" TextMode="Date" ></asp:TextBox>

var culture = CultureInfo.GetCultureInfo("es-SP");
var january = new DateTime(1, 1, 1).ToString("MMMM", culture); // enero
var thursday = new DateTime(2017, 12, 14).ToString("dddd", culture); // jueves