How should we make up for the fasts left due to menstruation? Is it obligatory to observe them later, and when?
Fasting is a worship that Allah has made obligatory in order to make us pious. It is a once in a year opportunity and therefore, Allah has made the Ramadan fasts compulsory for men as well as women. Having to leave these fasts due to menstruation should, in no way, be interpreted as a loss of bounties that could have been obtained. Women are required to make up for those fasts after the month of Ramadan. This should make them feel that they are not missing any part of that worship which ought to help them become pious.
One can ask why this rule is not the same for the prayer? The answer is very simple. Allah does not want to make His religion difficult for anyone. It is very easy to understand how difficult it would be to offer about thirty to fifty prayers each month, in addition to the five daily prayers.
You can keep the fasts that are left because of your menstruation whenever it is easy for you, but it is better to keep them as early as possible. At least, you should try your best to do this job before the start of the next Ramadan.