It was a nice day in Cloudsdale,and Rainbow Dash was preparing something with other pegasi. She eventually said:
-Ok,did everyone get what they're gonna do? Cloudchaser,you prepare the decoration. Spitfire,Soaring,practice for the special presentation. Fluttershy,you get the cake.
It seens they were planning Derpy's birthday party. Derpy saw they were acting kinda strange,so she followed them to see what was going on.
First,she followed Cloudchaser to see her filling baloons and painting posters.
-What is she gonna do with these things?
The cross eyed pegasus asked herself.
Derpy then proceeded to follow Spitfire and Soaring,she eventually saw the wonderbolts in the sky,writing a message that said "Happy Birthday Derpy". Derpy didn't actually get it.
-I guess they're having a presentation today,and they're practicing.
She then went down to Ponyville,to see what Flutters was up to. Derpy followed Fluttershy up to Sugarcube Corner. From out side,Derpy heard a conversation.
-Oh,hello Fluttershy! What do you want now?
-Hey Pinkie,I'm looking for a cake.
-I have various cakes,chocolate cakes,fruit cakes,cheese cakes,cupca-
-No,I came for THE cake.
-Oh,the SPECIAL cake,sure!
Pinkie then placed a bubble stamped grey box on Fluttershy's back,as Fluttershy left the building,Derpy thought:
-Umm...What could be inside that box? The only clues I have is that it has my cutie mark stamped in it.
Derpy then went to Sweet Apple Acres,and saw Applejack was around,so Derpy went over to her and said:
-Hey there AJ!
-Hello Derpy,what do you want?
-Oh nothing,I'm kinda investigating. Have you seen Fluttershy carrying a grey box around here? I followed her over here to know what was going on.
Applejack then got scared and said:
-I think you went the wrong way,Fluttershy is not around here,nope,she isn't anywhere.
Derpy got suspicious of the fact that Fluttershy was in the barn,then,not too far away from there,Rainbow Dash opened a box of blueberry muffins and placed it on the ground,Derpy then smelled the pastries and left AJ there.
Dash then spoke to Applejack:
-Thanks Rainbow Dash,if it wasn't for you,Derpy would've found out about the surprise party.
-You should thank Pinkie,she made the emergency batch of muffins.
Derpy then flies back to Cloudsdale,to see no pony was around,so she went back to Ponyville,to see no pony was there either.
-Where is everyone?
The cross eyed pony asked as she walked over to Sweet Apple Acres. Derpy heard some conversasion from outside the barn.
-Ok everypony,you know what to do,when Derpy enter,we'll go "Surprise!"
-Alright Twilight,we know.
-Then we get dance and eat cake,right?
-Yes Pinkie,than we'll get to dance and eat cake.
Derpy couldn't 100% understand what they were saying,but Derpy entered anyway. The moment Derpy entered,all her friends jumped and screamed "Surprise!"
-Oh,you guys were planning a birthday party,it all makes sense! Cloudchaser was filling the ballons and making drawings for the party,the Wonderbolts were training for a special presentation,and the cake Fluttershy got at Sugarcube Corner,was my birthday cake!
-Check,check and check,you got it all right
All the ponies then proceeded to dance,eat cake and have a good day.