俗話說，新年新跡象。在人們的觀念里，隨著新年的到來，周遭事物也跟著煥然一新，為我們帶來希望和快樂。由于新年伊始是一月，這種樂觀心理也因此得名“一月心情”。

January feeling refers to an emotional state characterized by feelings of optimism and possibility, particularly at the start of a new year.
“一月心情”是指新年伊始之際，對一切抱持樂觀態度，認為所有事都充滿各種可能性的心理狀態。

For example:

Is it just me or are you all looking forward to the end of 2015? Something about that fresh new January feeling — all things seem possible!
是只有我一個人有這種感覺，還是你們大家都在期盼迎來2015年年末？這是某些關于全新的“一月心情”，即一切似乎皆有可能！

There's another sense of this expression that means almost the opposite: An emotional state characterized by feelings of melancholy and lethargy, particularly early in the year when the holidays are over and a long winter looms. But this sense is quite old, dating to at least the mid 1920s.
該詞還有幾乎相反的另一種用法：指人們處于憂郁悲傷、無精打采的心理狀態，尤其當人們年初休完假，仍處于漫長冬季的時候。不過，這種用法較為陳舊，至少可追溯到十九世紀20年代中期。

For example:

Anyone who wants to lose "that January feeling" and substitute for that exhiliration and reinforced vitality, I recommend to go to the Exhibition of Canadian Art, open at present at the City Art Gallery.
我推薦那些想擺脫“一月心情”，恢復幸福快樂、生機盎然的狀態的人去參觀目前正在城市美術館展出的“加拿大藝術展”。

（中國日報網英語點津 丁一）