Yet Another Use of the Oblique Case

The oblique plural can be used in an idiom that indicates a repetition or succession:

 

दिनों दिन – “day after day”

वह दिनों दिन कुछ न कुछ पढ़ता रहा – “He kept studying something or the other day after day”

सालों साल – “year after year”

रातों रात – “all night long / overnight”

Note that the particular expression रातों रात actually means “overnight” or “all night long”:

वह रातों-रात करोड़पति बन गया – “He became a millionaire overnight”

उन्होंने रातों रात पूरी दूकान साफ़ कर दी – “He spent the whole night cleaning the entire shop”