Follow the below steps:

1. Add the below code in theme stlye.css

a.calendar {
    font-family: Tahoma, sans-serif;
    background-color: #ff0000;
    color: #ffffff;
    border-style: solid;
    border-width: 0px;
    right: -1px;
    cursor: pointer;
    -webkit-transform-origin: 100% 100%;
    -moz-transform-origin: 100% 100%;
    -o-transform-origin: 100% 100%;
    -ms-transform-origin: 100% 100%;
    -webkit-transform: rotate(-90deg);
    -moz-transform: rotate(-90deg);
    -ms-transform: rotate(-90deg);
    -o-transform: rotate(-90deg);
    transform: rotate(-90deg);
    -moz-border-radius-topright: 10px;
    border-top-right-radius: 10px;
    -moz-border-radius-topleft: 10px;
    border-top-left-radius: 10px;
    position: fixed;
    margin: 0;
    padding: 6px 13px 8px 13px;
    text-decoration: none;
    text-align: center;
    font-size: 15px;
    font-weight: normal;
    border-style: solid;
    display: block;
    z-index: 100000;
    top: 35%;
}

2. Add the below button code in theme functions file

add_action( 'wp_footer', 'wp_footer2');
function wp_footer2(){
<a class="calendar" href="/calendar">Calendar</a>
}