Date periods based on first occurence - list

Question

I have a pandas data frame of orders:
OrderID OrderDate Value CustomerID
1 2017-11-01 12.56 23
2 2017-11-06 1.56 23
3 2017-11-08 2.67 23
4 2017-11-12 5.67 99
5 2017-11-13 7.88 23
6 2017-11-19 3.78 99
Let's look at customer with ID 23.
His first order in the history was 2017-11-01. This date is a start date for his first week. It means that all his orders between 2017-11-01 and 2017-11-07 are assigned to his week number 1 (It IS NOT a calendar week like Monday to Sunday).
For customer with ID 99 first week starts 2017-11-12 of course as it is a date of his first order (OrderId 6).
I need to assign every order of the table to the respective index of the common table Periods. Periods[0] will contain orders from customer's weeks number 1, Periods[1] from customer's weeks number 2 etc.
OrderId 1 nad OrderId 6 will be in the same index of Periods table as both orders were created in first week of their customers.
Period table containig orders IDs has to look like this:
Periods=[[1,2,4],[3,5,6]]

Answer 1

Is this what you want ?
df['New']=df.groupby('CustomerID').OrderDate.apply(lambda x : (x-x.iloc[0]).dt.days//7)
df.groupby('New').OrderID.apply(list)
Out[1079]:
New
0 [1, 2, 4]
1 [3, 5, 6]
Name: OrderID, dtype: object
To get your period table
df.groupby('New').OrderID.apply(list).tolist()
Out[1080]: [[1, 2, 4], [3, 5, 6]]
More info
df
Out[1081]:
OrderID OrderDate Value CustomerID New
0 1 2017-11-01 12.56 23 0
1 2 2017-11-06 1.56 23 0
2 3 2017-11-08 2.67 23 1
3 4 2017-11-12 5.67 99 0
4 5 2017-11-13 7.88 23 1
5 6 2017-11-19 3.78 99 1