This year has 53 Sundays.
The three-year Sunday lectionary system (after Vatican II) is organised for 53 weeks – as this year has. If there are only 52 weeks, then one week is dropped out.
We stop counting the weeks when Lent begins (Ash Wednesday) and start counting again when Easter ends (The Day of Pentecost). This counting of weeks is called “Ordinary Time” (with the word ‘Ordinary’ connected to ‘ordinal’ – counting). If a week is skipped (in a 52-week year) it is dropped in that Lent/Easter window.
So this year, the week that led into Lent was “Week 8”. Now, because there are 53 weeks, no week is skipped and, hence, the week following the Easter Season is “Week 9”. The Day of Pentecost replaces the Sunday of Week 9. Just as, next week, Trinity Sunday will replace the Sunday of Week 10.
So, if you use a weekly collect on the non-feast weekdays this coming week, that is the collect for Week 9 (commentary here):
your providence is unfailing,
remove from our path, we humbly pray, whatever is harmful
and grant us those things that work for our good;
through Jesus Christ, our Saviour
who is alive with you,
in the unity of the Holy Spirit,
one God, now and for ever.
This also explains why the weekday (eucharistic) readings are those for Week 9.