month.html.erb

<% start_date = date.beginning_of_month.beginning_of_week - 1.day  %>
<% end_of_month = date.end_of_month.end_of_week  %>
<% weeks = (start_date...end_of_month).each_slice(7).to_a %>

<table>
  <thead>
    <% (0..6).each do |i| %>
    <th><%= (start_date + i.days).strftime('%a') %></th>
    <% end %> 
  </thead>
  <tbody>
    <% weeks.each do |week| %>
    <tr>
      <% week.each do |day| %>
      <td>
        <div><%= day.strftime("%-d") %></div>
      </td>
      <% end %>
    </tr>
    <% end %>
  </tbody>
</table>