Different perspectives on determining the first day of the months in Islamic calendar especially in Ramadhan and Syawwal bring complex problems for Muslims in Indonesia. Such different perspectives are mainly due to different understandings of Islamic jurisprudence (Fiqh) and political point of views. The major consequence for this month beginning difference is the inconcurrency times of doing prayers so that the harmony and confraternity of Muslims as a whole society is disrupted.For Indonesians whom the majority is Muslims, dealing with this problem of inconcurrency requiress long time and until recently it has not come to terms yet. As a matter of fact, this concern is crucial to be solved not only for Muslims in Indonesia but also and in the world. In the Indonesian context, the root of this problem is dissimilarity in possitionality among Indonesian Islamic organization. In dealing with this problem, the Indonesian government has tried to unite the Islamic calendar.Efforts such as doing researches, conferences, trainings, workshops, and even meetings that are aimed to facilitate unity in calendar have been undergone.However, the Indonesian government has not successfully united the calender. This is because of a common critiqueamong different Muslims group to the Indonesian Ministry of Religionon its inability to accomodate different people aspirations. Consequently, there are numbers ofIndonesian Muslims in Indonesia who do not follow the result of annual itsbat meeting held by government's.Based on the above description, the author focuses this paper on investigating Islamic law in terms of Muslims obligation to their government, especially in Itsbat meeting decision. To begin the discussion, the author provides the origin of different opinions among people in groups or Islamic organizations in deciding the start of qamariyah month in Indonesia. Afterwards, verses in Qur'an and Hadist that address Muslim's obligation in obeying legal government especially on virtuous deeds will be cited.