When a priest retires, are they welcomed back in their parish church or do they have to "stay away" for a given time. If so, how long, and why? Thanks.
I can't speak for every diocese because every diocese likely has their own policy and every diocese is likely going to take an issue like this on a case by case basis.
In my diocese when a priest retires the expectation is that they will move out of their parish and live somewhere else. If they want to remain in active ministry they would be given an assignment as a "Senior Priest" at another parish. Being a "Senior Priest" is like being an assistant pastor just with a fancier title and a little more "control" over what work you will do and not do. It is like a semi-retired role and for that reason a senior priest can negotiate with the pastor over the job description. An assistant pastor would not have that freedom.
If they want to fully retire and they live close to their former parish, and if the next pastor needs mass help he may ask the retired pastor to assist him. That however would be the new pastor's prerogative. He would be under no obligation to ask him or even allow him to say Mass in his Church.
I am sure you can see why having a retired pastor around MIGHT cause problems. What if the new pastor wants to move the parish in a different direction than the former pastor? What if the priorities of the new pastor are totally different from the priorities of the retired pastor? This can set up a difficult situation if the retired pastor doesn't agree with the pastoral priorities of the new pastor or the direction he wants to move the parish. It would have the potential to cause division whereby the retired pastor would have his supporters and the new pastor his supporters.
A situation like that could be analogous to having George Bush live in the Obama Whitehouse and work with the Obama administration.
Priests are like anyone else: they don't necessarily always agree with the pastoral priorities of other priests or the direction other priests want to take their parishes. Priests have different philosophies that guide their ministry and their pastoral priorities. More liberal priests tend to emphasize activism and social justice type things while more conservative priests tend to emphasize doctrine and teaching. More liberal priests tend to have less Roman ritualism in the Mass, less formal vestments and more contemporary music and contemporary instruments, while more conservative priests tend to have more roman ritualism, more formal music and more formal vestments. I hate to generalize----and "categorize" people but sometimes it is necessary in order to explain a point. In this case it is necessary to try and explain why it would not necessarily be a good thing to have a retired pastor hanging around his former parish.
If you have a retired pastor hanging around his former parish it can prevent the people from fully accepting the leadership of the new pastor, it can prevent the people from seeing their new pastor as their pastor and thus getting to know him, and it could lead to division.
Because of this--it is just better to have a retiring pastor move out of his former parish. Besides: such a thing is good for the people--because they understand that their former pastor is retired and has in a sense begun the next phase of his life---and because of this his relationship to the people has changed. He isn't their pastor anymore and is not in active ministry anymore. He has retired--and is now free to pursue his hobbies, interests, etc, as any retired person is.
For that reason most dioceses would have a policy in place that a retired pastor must move out of his former parish and have little involvement with the place--aside from mass help from time to time. That being said---depending on the personalities involved sometimes a retired pastor could remain---if the pastor agrees with it and the diocese supports it--but those cases would be less common.
I hope this explanation helps!