simple_calendar or DIY

1. gem
   gem simple_calendar

2. scss
   include the default stylesheet for the calendar in your app/assets/stylesheets/application.css file

   *= require simple_calendar
   

   highlight start-date in app/assets/stylesheets/simple-calender.scss

   .simple-calendar {
     .start-date {
         background-color: coral
     }
   }
   

3. views

   <%= month_calendar start_date_param: :date  do |date| %>
     <%= date.day %>
   <% end %>