"It was 2000 when we first met".
A preposição de tempo, quando nos referimos a ano, é "in", portanto "in 2000".
We first met in (the year) 2000
. (Informal English)
It was (the year) 2000 when
we first met. (Formal / Written English)
It was (the year) 2000 in which
we first met. (Formal / Written English).
A sentença com "in which" soa estranha porque é mais usada no inglês escrito ou formal.
Exemplo de um artigo de jornal:
(Year 2011) "It was a year in which
a significant earthquake struck Washington, D.C., yet failed to destroy a single federal agency. It was a year in which
the nation was subjected to a seemingly endless barrage of highly publicized pronouncements..."
Read more here: http://www.miamiherald.com/2011/12/31/v ... rylink=cpy