Wa alaikum salaam
When the sunsets, you can break your fast. The Athan of maghrib announces this.

Assalamualaikum sheikh, I have started to fast on the white days (13,14, 15) of every islamic month and also on mondays and thursdays. Whenshall I open the fast because if I follow the iftaar time calculated byscholars in my region it coincides with the azaan of magrib salah. What should I do in this case?