"It's good time to have Halloween party" would be more appropriate for this situation. No need for casual sentences "it would be timely."

"It will be timely." This would be an unnecessarily fancy way for most people to talk everyday, for something as informal as a Halloween party. Maybe the Queen of England would have said that.

All of the options you provided convey the idea that having a Halloween party is appropriate because Halloween is approaching. However, the most commonly used and natural-sounding way to express this idea is: "3. It is a good time to have a Halloween party." This sentence is clear and straightforward, and it effectively conveys your intention to have a Halloween party because the timing is right with Halloween approaching. It combines the notion of "good timing" and "Halloween party" in a clear and concise manner. Option 2, "It is good timing (because Halloween is in two weeks)," is also correct, but it's a bit more detailed and formal. Option 1, "It will be timely," works, but it might sound slightly less direct and more abstract. Option 4 would depend on the specific context and what you want to emphasize, but options 1, 2, and 3 are generally good choices for expressing your friend's idea.