As to why they are celebrated on the same day, legend has it that they were executed on the same day under the command of Emperor Nero and buried in Rome. (Although historical accounts cite Peter being martyred in 64 AD and Paul in 67 AD.)
Because of his Roman citizenship, Saint Paul was beheaded. St. Peter was reportedly crucified upside down because he said he was not worthy to be sacrificed in the same manner as Christ.
June 29, the Church celebrates the feast day of both men, and as early as 258 AD, there is evidence of an already-lengthy tradition that both Saint Peter and Saint Paul were celebrated on the same day.
In a sermon in the year 395, St. Augustine of Hippo said of Saints Peter and Paul:
“Both apostles share the same feast day, for these two were one; and even though they suffered on different days, they were as one. Peter went first, and Paul followed. And so we celebrate this day made holy for us by the apostles’ blood. Let us embrace what they believed, their life, their labors, their sufferings, their preaching, and their confession of faith.”