How about
Excel 2013/2016
A B C 2 Mon 10 Wed 3 Tue 15 4 Wed 20 5 Mon 15 6 Tue 10 7 Wed 20
<colgroup><col><col><col><col></colgroup><thead>
</thead><tbody>
</tbody>Cover
Worksheet Formulas
Cell Formula C2 =INDEX(A2:A7,MATCH(MAX(B2:B7),B2:B7,0))
<thead>
</thead><tbody>
</tbody>
<tbody>
</tbody>
If you dont like CTRL-SHIFT-ENTER then maybe this:
=LOOKUP(2,1/((MAX(INDEX(SUMIFS(B1:B6,A1:A6,A1:A6),0))=SUMIFS(B1:B6,A1:A6,A1:A6))),A1:A6)