Explanation
This is a jukugo word, which usually means on'yomi readings from the kanji. 月 has two on'yomi readings, but it always uses the がつ reading in the names of the months, so here's a mnemonic to help you remember that:
February is the month when somebody gives you a knee in the guts (にがつ). A complete stranger comes around each February just to knee you right in the guts. Ouch! That's why February is your least favourite month.