How to tweak formula to change calendar to include date range

Hello,

I've canibalised another sheet already created for a different user by Pete_UK, attempting to make it work for my own project.

Can someone tell me how I might adapt the following code to be able to calculate for a date spread between two columns:

=IFERROR(INDEX('Event Log'!$B:$B,MATCH(G3&"_"&$C4,'Event Log'!$D:$D,0)),"")

If it's impossible to understand without context, attachment is attached.
See Calendar (2) tab for the canibalised version.