Thread: which case is вечером?

In the following two Russian examples, why does the 2nd example have -ом tacked on at the end of the word вечер? Has it changed case? If so, which case is it in in the 2nd example?

EVENING
1. (Добрый) вечер ~ good evening
2. (сегодня) вечером ~ today [in the] evening

Originally Posted by SoftPretzel

In the following two Russian examples, why does the 2nd example have -ом tacked on at the end of the word вечер? Has it changed case? If so, which case is it in in the 2nd example?

EVENING
1. (Добрый) вечер ~ good evening
2. (сегодня) вечером ~ today [in the] evening

"Добрый вечер!" is a greeting.
"Сегодня вечером" - ответ на вопрос "Когда?". Вечером в творительном падеже.

вечер-ом is Instrumental case

Thank you quartztwo! Your reply led me to find the location of my very question in my Russian grammar book which explained why this is the case that is used.