1) Monday should be fine.
2) Monday would be fine.
Should in sentence 1 is not the same use of should that we find in examples such as:
- What should I do?
- You should apologise.
- You should eat five portions of fruit and vegetables every day.
The sentences above are asking for advice or giving advice. We can think of advice here like a form of weak obligation. If someone gives us advice, there is some pressure on us to do that thing - but we don't have to do it - it is our decision. This kind of meaning, when we talk about obligation and permission, is called DEONTIC modality.
Sentence 1 is NOT about deontic modality. This type of should is about EPISTEMIC modality. Epistemic modality is about knowledge and belief. Think about the following sentences:
- It might be in box 3.
- It should be in box 3.
- It must be in box 3.
The first example above shows that the speaker has a weak belief that it is in box 3. The second sentence shows that the speaker has a fairly strong belief that it is box 3, but she's not certain. The last example shows that the speaker is certain that it is in box 3.
In the Original Poster's example, (1) indicates that the speaker has a strong conviction, a strong belief, that Monday will be fine. Of course this is the technical meaning of what they are saying. The effect of saying this sentence is probably "Yes, choose Monday".
Would in sentence (2) indicates a logical result of choosing Monday. The sentence is like the last part of a conditional:
- If you chose Monday, Monday would be fine.
Here the speaker is definite about the fact that the result of choosing Monday is it's being fine. They aren't indicating any doubt about it. Again the effect in the conversation is probably "Yes, choose Monday".
Hope this is helpful!