Simplify the following text for a Grade 7 native English-speaking person.
Original: In 321 , Constantine instructed that Christians and non-Christians should be united in observing the venerable day of the sun , referencing the esoteric eastern sun-worship which Aurelian had helped introduce , and his coinage still carried the symbols of the sun-cult until 324 .
Simplified: In 321 , Constantine said that Christians and non-Christians should all join the `` day of the sun '' ( the eastern sun-worship which Aurelian had helped him introduce ) . His coins also had symbols of the sun-cult until 324 .