I am assuming you have the table t and then you want to know first sampled data of the week. As far as I know there is no function to directly solve this, you might need to apply a brute force way, I have a reference script for you. it is unoptimised but you can refer and create one for your use case.
idx = ;
uni = unique(t.w);
tt = 1;
idx = [idx i];
final_idx = ;
final_idx = horzcat(final_idx,find(t.i==t.i(idx(i)))');
desired_output = t(final_idx,:);
I hope this helps.