明天 is the next day after the speaking time. For example,
1) 现在 我 很忙， 明天 再 和你 聊天
xiànzài wǒ hěnmáng míngtiān zài hénǐ liáotiān
Now I am busy tomorrow with you chat
I am too busy now, will chat with you tomorrow
‘Speaking time’ means the moment that the speaker speak, and 明天 is the next day after it. Under this circumstance, we would use 明天 rather than 第二天
第二天 is the next day after the reference time. For example,
2) 上个 星期 我 之所以 没 参加 party，
shànggè xīngqīrì wǒ zhīsuǒyǐ méi cānjiā party
Last Sunday I the reason didn't join party
The reason why I didn't join in the party
是 因为 第二天 有 会议
shì yīnwéi dìèrtiān yǒu huìyì
is because the next day have meeting
is because I had a meeting the next day
'Last Sunday' is the reference time rather than the speaking time, and 第二天 is used after the reference time.
