Pentecost is the 50th day after Easter, and is known as Whitsunday. As Easter moves every year, so do all the occasions based on times before and after Easter are different every year, as is common in religions. If you know the year, it is easy enough to find out when date was Easter for example and work out the other religious events from then.
In Western Christianity, Advent begins on the fourth Sunday prior to Christmas Day, or the Sunday which falls closest to November 30, and lasts through Christmas Eve, or December 24. When Christmas Eve falls on a Sunday, it is the last, or fourth Sunday of Advent.
So, the 5th Sunday after Advent would also be the 1st Sunday after Christmas...
Epiphany is traditionally 6th January so the first sunday after 6th Jan would be the First Sunday after Epiphany and so on.
Trinity Sunday is the 1st Sunday after the Pentecost, which as we know is 7 Sunday's after Easter... so the 7th Sunday after Trinity would be the 8th after Pentecost and the 15th after Easter.
So check the calendar to find out when Easter was for your relevant year, and count forward 15 weeks and you should have your answer!