Star Trek: First Contact was released while the television series Star Trek: Deep Space Nine was still running. When does the film take place in conjunction with the series. I know that it should take place in the fifth season, but is it during any specific episode? Are the events ever mentioned in the series, since Worf was in both?
Star Trek: First Contact takes place on stardate 50893.5, while the DS9 5th season episodes "Children of Time" and "Empok Nor" take place on stardates 50814.2 and 50901.7 respectively, so according to the chronology the Borg invasion happened between these two.
The Borg attack was referenced in the episode "In Purgatory's Shadow" during a discussion between Sisko and Kira:
"A Dominion invasion of the Alpha Quadrant will affect Cardassia every bit as much as it's going to affect us. Besides, we need all the help we can get. The Dominion picked a perfect time to invade. The Cardassian fleet is in shambles, the Romulans are not much better off, and between the Klingon war and the recent Borg attack, Starfleet's spread pretty thin."