The simple meaning ("pshat") of the oral tradition is "They were unfaithful to the LORD and the resulting births were illegitimate. They will be consumed in a single month, together with their possessions".
So what's the problem? It starts with "chodesh", which can mean
- "month" referring to some particular month such as next month, or the month of tamuz
- "a month" meaning a span of time, about 28 days
- "the new moon festival"
Reading verse 7 alone, "hodesh" seems to mean "a month". But then verse 8 starts with "Blow the ram's horn in Giva'a", that sounds curiously like an echo of Psalms 81:4 "Blow the ram's horn on the new moon", but with an inverted meaning. Instead of a joyous festival, a festival of destruction. Since the prophets loved to make this type of inversion, verse 8 lends some credence to the NIV translation of verse 7.
"Them and their fields" - the Heb. is "chelkam", their portions of the land of Israel, or inheritances, but used here in the more comon sense of their worldly portions, possessions and capital investments. In verse 6 they came with just their sheep and cattle to sacrifice to the LORD but did not find him. And here in 7 they lose not only their sheep and cattle but the whole kit and caboodle. Note the alliteration in that the end of 6 is "yimtsaoo chalats meyhem" and the end of 7 is "chodesh et chelkehem", that is, a very similar set of letters in nearly the same order in a different set of words. It's like saying "you and your little dog too".
Have mercy on your translators. They are only allowed one verse per original verse but the prophets absolutely love double entendre, word plays and echoes of other passages from the tradition. These enrich the reading in the original but make simple translation impossible. In some cases each translator has something to bring to the table.