If the 1st day of Pesach were to occur on Sunday, I noticed that the fast of Baha"b (Some have a custom to fast on the 1st Monday, 1st Thursday and 2nd Monday following Rosh Hodesh Iyyar) would occur on the following dates:
- Monday 7 Iyyar
- Thursday 10 Iyyar
- Monday 14 Iyyar which is Pesach Sheni
Some people have the custom of reciting selichot on Ta'anit Behab. However, Pesach Sheni is a joyous holiday, in some sense. It was the day that people who were tameh (ritually impure) on Pesach and couldn't make the Pesach sacrifice, a 2nd chance to eat that sacrifice on 14 Iyar. Today, some have the custom to omit Tachanun on Pesach Sheni, a custom that is often done on holidays.
So there seems to be a contradiction between the joy of Pesach Sheni and the fasting on Beha"b. Since Pesach Sheni was a Biblical holiday, and Beha"b is a minhag, if the 2nd Monday of Beha"b is on Pesach Sheni, is fasting cancelled or postponed?