function show_summer() {
  var c_summer = document.getElementById('box_open_summer');
  var c_winter = document.getElementById('box_open_winter');
  var a_tab_summer = document.getElementById('a_tab_summer');
  var a_tab_winter = document.getElementById('a_tab_winter');  
  
  c_summer.style.display = "block";
  c_winter.style.display = "none";
  a_tab_summer.className = "boxtab_selected";
  a_tab_winter.className = "boxtab";

}

function show_winter() {
  var c_summer = document.getElementById('box_open_summer');
  var c_winter = document.getElementById('box_open_winter');
  var a_tab_summer = document.getElementById('a_tab_summer');
  var a_tab_winter = document.getElementById('a_tab_winter'); 
  
  c_summer.style.display = "none";
  c_winter.style.display = "block";
  a_tab_summer.className = "boxtab";
  a_tab_winter.className = "boxtab_selected";
}