St. Peter and St. Paul are two of the most well known saints, and are often spoken of as the two men most responsible for spreading the Christian message in the days of the early church.
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 the 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, St. 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.
On 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 St. Peter and St. 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.”
from Mike Hayes and the Busted Halo Question Box