You can compare the calendar situation on chart 100 by Imray. It shows the average pressures in July and December as well as a couple of other months.
The ITCZ is supposed to move by at least 5 degs and many sources claim more than this. The top limit of NE trades is supposed to move by at least such an amount.
One must consider additional facts - like the bent of the N limit of the trades (close to the continent) and also the presence of nasty Low pressure systems in the winter - we not only want to stay with the trades but also away from the swell as much as possible.
And, as said before, with modern aids to meteorology never ever stick to the square rigged tall ships route unless, off course, the actual weather
situation strictly adheres to the STATISTICAL picture as seen on Pilot Charts
In a word - sail by what there is, not by what you know from the Pilot Chart. The Pilot Chart is there to do the planning, and it is great for just that.