JSFiddle - React, Tailwind, and code Playground

HTML

<table id="fixed_hdr2">
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<tr>
<th colspan="5" rowspan="2"></th>
<th colspan="31">december</th></tr>

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<th>mon</th><th>th</th><th>we</th><th>tu</th><th>fr</th><th>sa</th><th>mon</th><th>th</th><th>we</th><th>tu</th><th>fr</th><th>sa</th><th>mon</th><th>th</th><th>we</th><th>tu</th><th>fr</th><th>sa</th><th>mon</th><th>th</th><th>we</th><th>tu</th><th>fr</th><th>sa</th><th>mon</th><th>th</th> 
</tr>	

<tr>
<th>#</th>
<th>#</th>
<th>#</th>
<th>#</th>
<th>#</th>
<th>02</th><th>03</th><th>04</th><th>05</th><th>06</th><th>07</th><th>09</th><th>10</th><th>11</th><th>12</th><th>13</th><th>14</th><th>16</th><th>17</th><th>18</th><th>19</th><th>20</th><th>21</th><th>23</th><th>24</th><th>25</th><th>26</th><th>27</th><th>28</th><th>30</th><th>31</th> 
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	<td colspan="31">test</td>
	
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<td>First</td>

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<td>Another row</td>

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<td>A</td>
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<td>Another row</td>

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<td>A</td>
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<td>D</td>
<td>Another...

CSS

td, th {
		border: 1px solid #aaa;
    padding: 8px;
	}