If you want to look for brands based on when they plan their spending, start with a new search (click Advanced Search on the left side of the screen). Next select Brands. Scroll down under Filter Options until you see the link for Planning Period. By clicking this, you are able to restrict your results to Q1, Q2, Q3, Q4, or Continuous planners.
NB: you are not restricted to just one of these options. If you want to search for advertisers planning in the second half of the year, you can select both Q3 and Q4.