expression, na-adjective
as if two great things happened at once; as if Christmas and one's birthday came together
Idiomatic expression describing a situation where two very happy or fortunate events occur simultaneously, like the two busiest or most joyful times of the year coinciding. Often used with 忙しい or 嬉しい. The literal image is the Bon Festival and New Year arriving together.
今日は給料日で、しかも彼女の誕生日。盆と正月が一緒に来たようだ。
Today is payday and also my girlfriend's birthday. It's as if Christmas and my birthday came at once.
新製品の発売と決算が重なって、盆と正月が一緒に来たような忙しさだ。
With the new product launch and the financial closing overlapping, it's as busy as if the two busiest days of the year came together.
From the image of the Bon Festival (盆) and New Year (正月), two of the busiest and most important holiday periods in Japan, arriving at the same time. The phrase is used figuratively to describe a coincidental convergence of happy or hectic events.