Here are a few possible interpretations depending on what the rest of the sentence could be:
1. "I will be in Nantes on Monday and..."
This assumes the missing part of the sentence is something like "I will be in Nantes on Monday and Tuesday" or "I will be in Nantes on Monday and going to [another place] on Tuesday."
2. "I am in Nantes on Monday and..."
This assumes the missing part of the sentence is something like "I am in Nantes on Monday and I will be in [another place] on Tuesday" or "I am in Nantes on Monday and I will be working/meeting/doing something on Tuesday."
3. "I am in Nantes on Monday, and..."
This is the most literal translation, but it's still missing information about what the speaker is doing or what is happening.
To understand the exact meaning, you need more context. For example, knowing the rest of the sentence, the speaker's intention, or the situation in which the phrase is used.