There is one mistake in each of the following sentences.

1. I have dancing lessons every Sarurday.
2. My birthday is on the 22nd of October.
3. Sally is ill. She is now in hospital.

Although you've said that only one mistake can be found in each of them, but I still find more than that.
1. I have dancing lessons every Sarurdays.
C: I have a dancing lesson every Saturday.
If 'every' is used, the following noun should be in singular, such as Saturday without a 's'. Also, there is only 'one' dancing lesson on every Saturday, but no 'dancing lessons'.

2. My birthday is on 22nd of October.
C: My birthday is on the 22nd of October.
Before the 22nd, it missed an article, the.

3. Sally is ill. She is now in the hospital.
C: Sally has been ill. She is now in the hospital.
Sally will be only in the hospital after getting sick. She will not be in the hospital when she suddenly felt ill. The pattern 'have + past participle' is used to discribe things occur without mentioning of time.

2007-10-13 19:42:42 補充：
For amendment, there is a missing of the preposition 'on' before the word 'every'.C: I have a dancing lesson on every Saturday.

2007-10-21 20:46:36 補充：
無聊。

1. I have dancing lessons every Sarurdays.
改: I have dancing lessons every Sarurday.

2. My birthday is on 22nd of October.
改:My birthday is on the 22nd of October.

3. Sally is ill. She is now in the hospital.
改:Sally is ill. She is now in hospital.