いきなりソースコード

こんなカレンダーができます。

calender.html

<!doctype html>
<html lang="ja"><head>
	<meta charset="utf-8">
	<style>
		/* 主要素 */
		html, body {
			margin: 0;
			padding: 0;
		}
		div {
			background: #00f;
		}
		div div {
			display: flex;
			flex-wrap: wrap;
			margin: 0 auto;
			padding:1vw;
			background:#e3e9ff;
			justify-content: center;
			font-size: 1.5vw;
		}
		/* 曜日のコンテナ */
		div div div {
			width: 11.9vw;
			height: 16.6vh;
			justify-content: right;
			padding: .5vh .8vw;
			margin: 0;
			border: .1vw solid #000;
			background: #fff;
		}

		/* 曜日記載部分 */
		div div div:nth-child(1),
		div div div:nth-child(2),
		div div div:nth-child(3),
		div div div:nth-child(4),
		div div div:nth-child(5),
		div div div:nth-child(6),
		div div div:nth-child(7) {
			color: #000;
			background: #e3e9ff;
			height: 4vh;
			justify-content: center;
			border: .1vw solid transparent;
			padding: .5vh .8vw;
		}
		/* 休日 */
		div div div:nth-child(1),
		div div div:nth-child(8),
		div div div:nth-child(15),
		div div div:nth-child(22),
		div div div:nth-child(29),
		div div div:nth-child(36) {
			color: #f00;
		}
		div div div:nth-child(7),
		div div div:nth-child(14),
		div div div:nth-child(21),
		div div div:nth-child(28),
		div div div:nth-child(35),
		div div div:nth-child(42) {
			color: #08f;
		}
		/* 予定表 */
		div div div span {
			width: 100%;
			display: block;
			color: #fff;
			background:#668cd9;
			height: 4.5vh;
			border-radius: 10% 10% 10% 10%;
			margin-bottom: 0;
			padding: 1%;
		}
	</style>
</head>
<body>
				<div>
					<div>
						<div>日</div>
						<div>月</div>
						<div>火</div>
						<div>水</div>
						<div>木</div>
						<div>金</div>
						<div>土</div>
						<div>30</div>
						<div>31</div>
						<div>11月1</div>
						<div>2</div>
						<div>3</div>
						<div>4</div>
						<div>5<span>5と0の付く日</span></div>
						<div>6</div>
						<div>7</div>
						<div>8</div>
						<div>9</div>
						<div>10<span>5と0の付く日</span></div>
						<div>11</div>
						<div>12</div>
						<div>13</div>
						<div>14</div>
						<div>15<span>5と0の付く日</span></div>
						<div>16</div>
						<div>17</div>
						<div>18</div>
						<div>19</div>
						<div>20<span>5と0の付く日</span></div>
						<div>21</div>
						<div>22</div>
						<div>23</div>
						<div>24</div>
						<div>25<span>5と0の付く日</span></div>
						<div>26</div>
						<div>27</div>
						<div>28</div>
						<div>29</div>
						<div>30<span>5と0の付く日</span></div>
						<div>12月1</div>
						<div>2</div>
						<div>3</div>
					</div>
				</div>
</body>
</html>

曜日処理の必要がなくなりました。

半面・・・祝日処理は必要です。

かわりに、div に対して nth-child をしまくっています。

最後にsvg版

calender.svg

<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 560 400" width="560" height="400">
	<style>
		div div {
			display: flex;
			flex-wrap: wrap;
			margin: 0 auto;
			padding:1%;
			background:#e3e9ff;
			justify-content: center;
			font-size: 1.5%;
			width: 560px;
			height: 400px;
		}
		/* 曜日のコンテナ */
		div div div {
			width: 11.9%;
			height: 17%;
			justify-content: right;
			padding: .5% .8%;
			margin: 0;
			border: .1% solid #000;
			background: #fff;
		}

		/* 曜日記載部分 */
		div div div:nth-child(1),
		div div div:nth-child(2),
		div div div:nth-child(3),
		div div div:nth-child(4),
		div div div:nth-child(5),
		div div div:nth-child(6),
		div div div:nth-child(7) {
			color: #000;
			background: #e3e9ff;
			height: 4%;
			justify-content: center;
			border: .1% solid transparent;
			padding: .5% .8%;
		}
		/* 休日 */
		div div div:nth-child(1),
		div div div:nth-child(8),
		div div div:nth-child(15),
		div div div:nth-child(22),
		div div div:nth-child(29),
		div div div:nth-child(36) {
			color: #f00;
		}
		div div div:nth-child(7),
		div div div:nth-child(14),
		div div div:nth-child(21),
		div div div:nth-child(28),
		div div div:nth-child(35),
		div div div:nth-child(42) {
			color: #08f;
		}
		/* 予定表 */
		div div div span {
			width: 100%;
			display: block;
			color: #fff;
			background:#668cd9;
			height: 15%;
			border-radius: 10% 10% 10% 10%;
			margin-bottom: 0;
			padding: 2%;
		}
	</style>
	<g>
		<foreignObject x="0" y="0" width="100%" height="100%">
			<div xmlns="http://www.w3.org/1999/xhtml">
					<div>
						<div>日</div>
						<div>月</div>
						<div>火</div>
						<div>水</div>
						<div>木</div>
						<div>金</div>
						<div>土</div>
						<div>30</div>
						<div>31</div>
						<div>11月1</div>
						<div>2</div>
						<div>3</div>
						<div>4</div>
						<div>5<span>5と0の付く日</span></div>
						<div>6</div>
						<div>7</div>
						<div>8</div>
						<div>9</div>
						<div>10<span>5と0の付く日</span></div>
						<div>11</div>
						<div>12</div>
						<div>13</div>
						<div>14</div>
						<div>15<span>5と0の付く日</span></div>
						<div>16</div>
						<div>17</div>
						<div>18</div>
						<div>19</div>
						<div>20<span>5と0の付く日</span></div>
						<div>21</div>
						<div>22</div>
						<div>23</div>
						<div>24</div>
						<div>25<span>5と0の付く日</span></div>
						<div>26</div>
						<div>27</div>
						<div>28</div>
						<div>29</div>
						<div>30<span>5と0の付く日</span></div>
						<div>12月1</div>
						<div>2</div>
						<div>3</div>
					</div>
				</div>
		</foreignObject>
	</g>
</svg>