When do we need to select concentration?

Hi Guys,
For US schools (like Wharton/CBS/Kellogg/Tuck/Ross/Duke/Darden/UCLA/McCombs etc) can you please let me know when do we need to actually select the electives at high level /concentration...? Is it at the time of submitting online application or after the one year of study of core courses at the school ?

Much appreciate any feedback on this.

P.S : Experts/Admission Consultants-it would be great if you can share your thoughts on the same.