Continuity mistake: During Does Your Mother Know one of the girls in the background has red sunglasses which she wears in one shot and doesn't in the next - it keeps changing.
Continuity mistake: When Lay All Your Love on Me starts, right before Sophie's first singing lines, she has some hair in front of her face. But, when it switches to a shot in front of her as she continues to sing, she immediately has no hair in front of her face. She didn't have the time to move it out of the way in-between shots.
Continuity mistake: During the song "The winner takes it all" at one point 4 people are walking up the mountain in the background, 2 in white and 2 in mint green. The next shot they are gone.
Answer: Sophie finds Donna's diary from 1979, being the year Sophie was conceived meaning she was born in 1980 (as the diary entries are from July/August). So the movie is set in 2000.