=IFERROR(INDEX... to match action items with due date

Hello all - I'm hoping one of your brilliant mind can help me out...I am trying to use a formula: =IFERROR(INDEX(P:P,MATCH(H$3&”_”&ROWS($1:1),S:S,0)),””) to search a row of due dates, run them against a due date in another cell and, if there is a match, return the associated action line that is in another column. I can't seem to get the function to work. Essentially, I'd like to auto-populate a calendar with action items (in a table/list form) by pasting action items into their "date" box.

Beyond this, if there is any possible way for the formula to capture both start and end dates (for those actions that span multiple days - ex. "develop first draft" vs one-off action items "strategy meeting") I'd love to populate both on the calendar and conditionally format to color the cells between the start/end dates.

I've attached a simplified excel file to show what I'm trying to accomplish. Thank you SO MUCH, in advance!!

Autopopulation of Action Items onto Calendar.xlsx

This works great on the short-list in the excel file I attached, however how would you recommend building the formula so that it could search through a long list (~100 items) to find any that match that day (ex. B3) and also be smart enough to extend the action across multiple columns if the start date differs from the end date? Thank you!!