Once you have completed the Apprentice Objectives, you can enter the grand prix.
It runs 5 days a week, and has both Classical as Western competitions

Any horse that is old enough can enter, with a maximum of 2 horses per competition (or 4 if you have the bonus that can be obtained by opening a Horn of Plenty)

once you have completed your 75th beginner objective you will unlock the grand prix tutorial. Just like you did when you unlocked the EC and the direct sales. Do the tutorial and Emma will show you how to enter.

Thank you so much for your help!

I have done 86 objectives done. How can I finish Appentice objectives when 2 of them are Grand Prix?

I feel silly. I no more posted that and Emma shows up about the Grand Prix. I guess I was a little impatient. Sorry about that.

Just a note as well, the prix is one of the four special features. One of the very final objectives is to activate them, which are the ones at the bottom of the objectives page.

Thank you very much