a lookup formular in convert my daily data into monthly data

Hi All,

I want to build a lookup formular in convert my daily data into monthly data.

There is a tricky part is here: some of my data does not start to have value on the first couple days of the month, but i want my lookup table to reflect a value of any day in the month

For example, In colume A, my data starts with 0 for the first of day in may 2026, and then it begins to have value. I want my lookup table in group B to reflect the cell with number on May 2026.


Many Thank