しりません actually means “to have no knowledge of something” but about tomorrow, you are likely to have some idea about what might be happening or what you may be doing. You just don’t know it for sure yet. If you say しりません in this situation, it sounds like you are refusing to talk about it any further and the asker will get quite often offended.
On the other hand, わかりません can imply “I don’t know it well enough to explain it” so the asker can still keep talking to you. That is the reason why わかりません is better in this situation.
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