The textbook Japanese for busy people 1 tells me, in lesson 6, that "relative time expressions like あした, らいしゅう, こんげつ and きょねん generally do not take any particles."
They exemplify this with sentences like:
One lesson later, they tell me that "unlike relative time expressions, specific time expressions take the particle に.
どようびに, 'on Saturday'
２００６ねんに, 'in 2006'"
They then provide these example sentences:
However, and contradicting the explanation given, the second example sentence does not take に. Furthermore, they do not provide any example as to why. Is there any rule that explains this? When do specific time expression take the に particle, and when don't they?
P.S.: I know no kanji, so please provide your answers in kana. Thank you.