Due to different lengths of your daily data you should use a cell array:
A = readmatrix('New Text Document.txt');
k_min = min(unique(A(:,1)));
k_max = max(unique(A(:,1)));
x = cell(1,k_max-k_min+1);
for k = k_min:k_max
x{:,k} = A(A(:,1)==k,2);
end
Access the data of the single days by using:
x{1,2} % to look day 2
x{1,5} % day 5
