Questions about certain words.
 

1.) If I'm trying to express yesterday night, can I use "昨日の晩" and "ゆうべ" interchangeably? Or does it depends on the grammar?

2.) The word "毎月", can be spelled using both　"まいげつ"　and　"まいつき", but which of two is more frequently used? Or does it also depends on the grammar?

3.) The character "さ" used in words such as ”さ来月”、”さ来年” appears to be "再" whenever I type it out, such as "再来月","再来年" and so on. Is there any rule regarding when should the Katakana characters be written in Kanji?

I often run into above situations when I'm trying to read short sentences, so I'm wondering whether or not there are some general guide lines to follow.

Thanks.

1. 「昨日の晩」 is not really used much as it sounds pretty wordy. We use 「昨晩」, which has the same meaning as 「ゆうべ」. The former is more formal than the latter.

Big Rule of Thumb:
Originally Japanese words such as 「ゆうべ」 are used in more informal situations than the Chinese-origin kanji words. That means we tend to use more kun-reading words in everyday conversations and use more on-reading words in formal/technical situations.

In English, you guys do just about the same thing with the big Latin-origin words.

2. Both are correct and interchangeable. However, MUCH more Japanese-speakers pronounce it as まいつき.　You would rarely hear まいげつ.　

3. No "rules"; Just tendencies. More people would write them with all kanji.

Well explained as usual, much thanks.