Conditional merge on 2 huge pandas datasets

Question

I have 2 dataframes as below:

A = pd.DataFrame({\'UserId\':[\'U1\',\'U1\',\'U1\',\'U2\',\'U2\',\'U3\'],\'End_Week\':[\'1/6/2020\',\'15/6/2020\',\'6/7/2020\',\