Here are the monthly winning fund counts from 2001 to 2021 ordered from most wins to least.
[TD="align: left"]Jan[/TD]
[TD="align: left"]8S[/TD]
[TD="align: left"]8F[/TD]
[TD="align: left"]2G[/TD]
[TD="align: left"]2I[/TD]
[TD="align: left"]1C[/TD]
[TD="align: left"]Feb[/TD]
[TD="align: left"]8S[/TD]
[TD="align: left"]4I[/TD]
[TD="align: left"]4G[/TD]
[TD="align: left"]3F[/TD]
[TD="align: left"]2C[/TD]
[TD="align: left"]Mar[/TD]
[TD="align: left"]9S[/TD]
[TD="align: left"]5C[/TD]
[TD="align: left"]3F[/TD]
[TD="align: left"]2I[/TD]
[TD="align: left"]2G[/TD]
[TD="align: left"]Apr[/TD]
[TD="align: left"]10I[/TD]
[TD="align: left"]4S[/TD]
[TD="align: left"]3C[/TD]
[TD="align: left"]3F[/TD]
[TD="align: left"]1G[/TD]
[TD="align: left"]May[/TD]
[TD="align: left"]11S[/TD]
[TD="align: left"]4F[/TD]
[TD="align: left"]4I[/TD]
[TD="align: left"]1C[/TD]
[TD="align: left"]1G[/TD]
[TD="align: left"]Jun[/TD]
[TD="align: left"]9S[/TD]
[TD="align: left"]5G[/TD]
[TD="align: left"]3F[/TD]
[TD="align: left"]3I[/TD]
[TD="align: left"]1C[/TD]
[TD="align: left"]Jul[/TD]
[TD="align: left"]6F[/TD]
[TD="align: left"]6S[/TD]
[TD="align: left"]4C[/TD]
[TD="align: left"]3I[/TD]
[TD="align: left"]2G[/TD]
[TD="align: left"]Aug[/TD]
[TD="align: left"]7F[/TD]
[TD="align: left"]7S[/TD]
[TD="align: left"]3I[/TD]
[TD="align: left"]2G[/TD]
[TD="align: left"]2C[/TD]
[TD="align: left"]Sep[/TD]
[TD="align: left"]8I[/TD]
[TD="align: left"]4F[/TD]
[TD="align: left"]4S[/TD]
[TD="align: left"]4G[/TD]
[TD="align: left"]1C[/TD]
[TD="align: left"]Oct[/TD]
[TD="align: left"]7S[/TD]
[TD="align: left"]5C[/TD]
[TD="align: left"]4I[/TD]
[TD="align: left"]4G[/TD]
[TD="align: left"]1F[/TD]
[TD="align: left"]Nov[/TD]
[TD="align: left"]11S[/TD]
[TD="align: left"]6C[/TD]
[TD="align: left"]2F[/TD]
[TD="align: left"]1G[/TD]
[TD="align: left"]1I[/TD]
[TD="align: left"]Dec[/TD]
[TD="align: left"]11I[/TD]
[TD="align: left"]5S[/TD]
[TD="align: left"]2G[/TD]
[TD="align: left"]2F[/TD]
[TD="align: left"]1C[/TD]
As you can see the I Fund wins in December over half the time while the S Fund takes January about 40% of the time.