默认显示当月日历，点击左右切换可以选择月份：

calender.png

HTML 部分：

<div class="box">

    <div class="bar">
        <div><span class="year"></span>年<span class="month"></span>月</div>
        <div>
            <span class="prev"><</span><span class="next">></span>
        </div>
    </div>
    <ul class="head">
        <li>日</li>
        <li>一</li>
        <li>二</li>
        <li>三</li>
        <li>四</li>
        <li>五</li>
        <li>六</li>
    </ul>

    <ul class="content">

    </ul>
</div>

css 部分：

*{
            margin: 0;
            padding: 0;
        }
        .box{
            margin: 0 auto;
        }
        .head, .content{
            list-style: none;
            display: flex;
            flex-wrap: wrap;
            border: 1px solid lightseagreen;
            width: 364px;
            margin: 10px auto;
        }
        .head li,.content li{
            list-style: none;
            width: 50px;
            height: 50px;
            text-align: center;
            line-height: 50px;
        }
        .head li{
            border: 1px solid lightseagreen;
        }
        .content li{
            border: dashed 1px lightgrey;
        }
        .content li.dark{
            background-color: #d3d3d3a1;
            color: darkgray;
        }
        .active{
            background-color: lightseagreen;
        }
        .bar{
            width: 364px;
            height: 30px;
            line-height: 30px;
            margin: 10px auto 0;
            display: flex;
            justify-content: space-between;
            align-items: center;
            font-size: 20px;
            font-weight: bold;
        }
        .prev, .next{
            padding: 0 20px;
            cursor: pointer;
        }

JS 部分：

$(function () {

        let content = $('.content');
        let totalDays;
        let now = new Date();
        let today = now.getDate();
        let thisYear = now.getFullYear();
        let thisMonth = now.getMonth() + 1;

        // 月份选择按钮
        $('.prev').click(function () {
            now.setMonth(now.getMonth() - 1);
            setCalender();
        });
        $('.next').click(function () {
            now.setMonth(now.getMonth() + 1);
            setCalender();
        });

        // 封装的生成日历方法
        function setCalender(){
            content.empty();
            let month = now.getMonth() + 1;
            let year = now.getFullYear();
            
            // 日历头上显示的月份
            $('.year').html(year);
            $('.month').html(month);

            // 判断每月天数
            switch (month) {
                case 1:
                case 3:
                case 5:
                case 7:
                case 8:
                case 10:
                case 12:
                    totalDays = 31;
                    break;
                case 4:
                case 6:
                case 9:
                case 11:
                    totalDays = 30;
                    break;
                default:
                    if((year % 4 === 0 && year % 100 !== 0) || year % 400 === 0){
                        totalDays = 29;
                    }else{
                        totalDays = 28;
                    }
            }

            // 将当月的天数加进日历
            for(let i = 0; i< totalDays; i++){
                let li = $('<li/>').html(i+1);
                if (i === today -1 && month === thisMonth && year === thisYear){li.addClass('active')}
                content.append(li);
            }

            // 上一个月的天数补上
            now.setDate(1);  // 日期设置为 1 号
            let firstDay = now.getDay();  // 获取当月第一天是星期几

            for (let i = 0; i < firstDay; i++) {
                now.setDate(now.getDate() -1);
                let li = $('<li/>').html(now.getDate()).addClass('dark');
                content.prepend(li);
            }

            // 下个月的天数补上
            now.setDate(now.getDate() + firstDay + totalDays -1);  // 日期设为本月最后一天
            let lastDay = now.getDay();  // 获取最后一天是星期几

            for (let i = 0; i < 6 - lastDay; i++) {
                now.setDate(now.getDate() + 1);
                let li = $('<li/>').html(now.getDate()).addClass('dark');
                content.append(li);
            }

            // 将日期设置为当月1号，因为每个月都有 1 号
            now.setDate(now.getDate()- 8);  // 月份切回当月
            now.setDate(1);
            console.log(now)
        }

        setCalender();

    })