[EDIT] 약간의 수정

react-whenMeet/src/components/MakeDay2.jsx

 export default function MakeDay2() {
   const days = [];
-    const date = ['Sun', 'Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat'];
+  const date = ["Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat"];
 
   for (let i = 0; i < 7; i++) {
-        days.push(
-            <th className="table_head">{date[i]}</th>
-        )
+    days.push(<th className="table_head">{date[i]}</th>);
   }
 
   return (
     <thead>
       <tr>
-            <th></th>
+        <th className="table_head"></th>
         {days}
       </tr>
     </thead>

react-whenMeet/src/components/MakeHeader.jsx

-import Button from "./Button";
-
 export default function MakeHeader({
   prevMonth,
   nextMonth,
@@ -7,9 +5,8 @@ export default function MakeHeader({
   nowYear,
 }) {
   return (
-    <div>
+    <div className="header">
       <h2>
-        <span className="header">
         <button type="button" onClick={prevMonth}>
           prev
         </button>
@@ -18,7 +15,6 @@ export default function MakeHeader({
         <button type="button" onClick={nextMonth}>
           next
         </button>
-        </span>
       </h2>
     </div>
   );