Find day values in bigint date?
Firstly, why on earth are you storing dates as BIGINT?! 

Date fields should be stored as dates, otherwise you could end up inserting invalid dates and you'll have trouble doing basic date calculations, as per now.

To answer your question, you'll need to CAST the InvoiceDate field, firstly to a VARCHAR, then to a DATE type:

SELECT DATENAME(WEEKDAY, CAST(CAST(InvoiceDate AS VARCHAR(10)) AS DATE)) AS [WeekDay]
    FROM Store;