A mistake

The first סעיף in סימן תיח states that is it prohibited to fast on Rosh Chodesh.

I wrote a previous post about Taanit Tzedek, a group of "rabbis" who describe themselves on a website as an ad hoc group of rabbis who are committed to fasting for the people of Gaza and calling for the lifting of the blockade..
Last week, I clicked on their site and noticed that they had called for a fast on Rosh Chodesh. (see above)
Today, I see they have changed the date to the day before Rosh Chodesh.
What precipitated the change? Could the rabbis have made a mistake in going against a Mishnah Berurah? If so, perhaps this whole fasting idea is a big mistake.