TAKOBOTO

To decide on, to choose
Let's make it Sunday.

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うちはまいとしくがつまつのにちようびにするときまっているのです。

It's our rule to make it the last Sunday of September every year.

そのようにするとあとでこまらない？

if you do it this way, won't you be troubled later on?

Dictionary of Basic Japanese Grammar p. 310

anon

The Japanese is not natural from the context. It should be 毎年9月の最終日曜日.
For example, if the Sunday was 26th, we don't say 9月末（９がつすえ）。
ちなみに、月末（げつまつ） is at the end of month. 下旬（げじゅん） is end of month.

Miki

9月末 is read ９がつまつ.
hmm but I found some sites 毎年○月末の▲曜日…。

Miki

I agree with Miki. It sounds more natural. If you don't like 最終日曜日、you can say 最後の日曜日.

bamboo4