Indian Summer Monsoon Rainfall (ISMR) experiences quasi-periodic drought and excess rainfall. In this study we examined the contribution of individual month's (June–September) rainfall in the seasonal excess (deficit). Analysis of 110 years (1901–2010) of observed precipitation and 20th century reanalysis data highlights the importance of subseasonal variations of rainfall in modulating Indian summer monsoon. A month which contributes to seasonal excess (deficit) is primarily controlled by large-scale La Niña (El Niño) forcing but with favourable local conditions from the tropical Indian Ocean. On the other hand excess (deficit) rainfall of an individual month which is not contributing to seasonal excess (deficit) is controlled mainly by the local forcing. Such local forcing however is short lived and does not persist more than a month. Our analysis reveals that June is not contributing considerably to the seasonal excess rainfall in the recent years. On the other hand contribution of September rainfall to the seasonal extremes is more frequent in the recent years. During September enhanced El Niño (La Niña) conditions and the local forcing contribute to the seasonal ISMR deficit (excess). It is important to note that none of the above 110 years experienced excess (deficit) rainfall during all the four monsoon months except 1972. This study advocates the need of subseasonal (or monthly) rainfall prediction for better socio-economic benefits.