I was struggling to translate the following poster in a rigorous manner, more specifically to Korean, which is a LOT easier than translating Japanese to English.
It says "2016年レギュラーシーズン公式戦の西武プリンスドーム開催試合においていずれも当日、残席がある場合のみ"
Now I have divided the phrase "-開催試合においていずれも当日" into
"-開催試合において" / "いずれも当日"
"Regarding Prince Dome Games" / "???? the day"
And from the context I've concluded that the phrase really means
"All Prince Dome games regardless of the day" and subsequently,
"The game is free only if it is a regular season game @ Prince Dome, and if there are free seats left."
Which sounds pretty reasonable, but I still can't really pinpoint the definition of the word "いずれも," and Googling didn't help at all.
I was thinking "all," but why the phrase when we have 全部 or すべて?