JSFiddle - React, Tailwind, and code Playground

HTML

<div class="month_block" data-idboard="id_1">show 1</div>
       <div class="main_calendar" data-idboard="id_1">map 1</div>

 <div class="month_block" data-idboard="id_2">show 2</div>
       <div class="main_calendar" data-idboard="id_2">map 2</div>

 <div class="month_block" data-idboard="id_3">show 3</div>
       <div class="main_calendar" data-idboard="id_3">map 3</div>

CSS

.main_calendar { display: none; }

JavaScript

$('.month_block').hover(function() {
    var idboard = $(this).attr('data-idboard');
    $('.main_calendar[data-idboard="' + idboard +'"]').slideToggle(200);
});