Will August be always summer in the northern hemisphere? Is the Earth orbit precessing, or are there other effects which will create a shift between our calendar (day counting), and the Earth's orbit?
I imagine these effects to be small, but I'm asking for long timescales.
[Edit] To formulate my question better, let me be more precise. Assume that our calendar never changes, meaning we keep counting days always in the same way (second...days defined by an atomic clock, 365 days = 1 year, usual leap years, etc), and consider what will happen in -I don't know- 100k-1M year? Or, if this timescale is wrong, what should it be to see an effect of a shift between seasons and months, as we are used to?
 A: 
A sidereal year is the time taken by the Earth to orbit the Sun once with respect to the fixed stars. ...
  The sidereal year differs from the tropical year, "the period of time required for the ecliptic longitude of the Sun to increase 360 degrees", due to the precession of the equinoxes. The sidereal year is 20 min 24.5 s longer than the mean tropical year

Our Gregorian calendar is based on the tropical year, so it has the seasonal drift "built in" to it. (This has nothing to do with leap days. They are needed because the tropical year is not an integer number of days.)
A: The Gregorian calendar, the calendar used most, adds a leap year every 4 years, and skips 3 leap days every 400 years. This gives it an average year of 365.2425 days. It has been proposed to omit another leap day every 4000 years to keep the Gregorian calendar even closer to an astronomers mean tropical year, currently 365.24219 days.
A: The seasons are caused by the tilt of the Earth's axis of rotation. (The northern hemisphere has summer when it is tilted toward the sun.)  Because of the equatorial bulge, the moon exerts a periodic torque on the earth which causes the axis to precess (once every several thousand years). The seasons will shift relative to the year as defined by orbital motion.
