Python: How to efficiently do operations using different rows of the same column?
If you have unique date or DaysToReception, you can actually use Map/HashMap where the key will be the date or DaysToReception and the values will be other information which you can store using a list or any other appropriate data structure. 
This will definitely improve the efficiency. 
As you pointed out that "number of rows I search the value below depends on the value "DaysToReception", I believe "DaysToReception" will not be unique. In that case, the key to your Map will be the date.
My goal is that by given  a value on a row (let's say 3), look for the value of a given column 3 rows below. Currently I am perfoming this using for loops but it is tremendously inefficient.
I have read that vectorizing can help to solve this problem but I am not sure how.
My data is like this:
Date DaysToReception Quantity QuantityAtTheEnd
20/03     3             102
21/03     -             88
22/03     -             57
23/03     5             178
24/03
And I want to obtain:
Date DaysToReception Quantity QuantityAtReception
20/03     3              102         178
21/03     -              88
22/03     -              57
23/03     5              178
24/03
...
Thanks for your help!