Is it a custom or law to have special meal on Rosh Chodesh? I had never heard of this.
Thank you for your question.
There is no obligation to specifically have a meal, owever it is a good thing to add something special to the meal in honor of Rosh Chodesh. The gemora says that the added expenses that a person incurs spending for the honor of Rosh Chodesh are paid back by Hashem, the same way the expenses for Shabbos and Yom Tov are.
Have a good Chodesh
Vayikra Rabba 17,ORach Chaim 419, M:B 419-1.