A year ago, I was leading the services in shul on the Shabbos before Rosh Chodesh (Mar)Cheshvan. As is the custom on the Shabbos before Rosh Chodesh, a short prayer is inserted wherein the time of the New Moon is announced, as well as which days are going to be celebrated as Rosh Chodesh. As a part of the announcement, the name of the month is mentioned. And so, when I was ready to make the announcement, I began "Rosh Chodesh Cheshvan...." Immediately I was stopped and asked to announce it as MarCheshvan.
Mar, of course, means "bitter" in Hebrew. The most common explaination given for the addition of this prefix to the name of the month is because Cheshvan is devoid of Jewish holidays. As such, it is deemed to be "bitter."
Personally, this has always troubled me. After all Elul has no holidays either. But let's write off Elul by saying that it's swept up in the whole pre-Rosh HaShannah hype. But certainly Cheshvan is no worse off than Tammuz. Tammuz also has no holidays and, in fact, has a major fast day. At Cheshvan has no negatives at all! So, why do people refer to Cheshvan as MarCheshvan but not to Tammuz as MarTammuz?