For the tango tourist in Buenos Aires there are some very useful free apps to ease your life.
The first one I recommend you is "Milonga Hoy" (or "Hoy Milonga", depending if it's iOS or Android). You can download this app in your normal app store.
Milonga Hoy gives you an overview about all milongas in Buenos Aires (Capital Federal) per day. It lists you the names, addresses, times, numbers of colectivos and metros running close, the names or organizers, Facebook or web pages, phone numbers for table reservations, and more information about the milonga. Additionally you can see a map of the city with all the locations, so you can choose the ones close to the area you live in.
Usually the information for Milonga Hoy in Buenos Aires are quite exact, but there are exceptions. It is always better to double-check the information if you happen to go for the first time to a milonga, or are otherwise not sure. Most Milongas you can find meanwhile with their own Facebook page.
The app lists all milongas for the actual day, as well as for the upcoming week.
They have as well a website with the same name and the same services, you can find it here