Namaz is a daily, five-time obligation. Because it is so frequent, even a small delay can cause missed prayers to pile up very quickly. However, a single prayer usually only takes 5 to 10 minutes. Because the time commitment is so small, it is much easier to fit into a busy day “as soon as possible” without disrupting your basic human needs or responsibilities.
Fasting is a completely different kind of commitment. It is physically demanding and requires you to dedicate an entire day (from dawn to sunset) to the act. It requires good health, stamina, and a specific mental state (niyat). Because it is a 14–16 hour commitment, the Shariat naturally allows more flexibility in when you choose to fulfill it.
Islam is a religion of ease and moderation. While “earliest is best” remains the ideal (simply because life is unpredictable), it is not meant to come at the price of your well-being or your duties as a guest or family member.
The “earliest” requirement is a spiritual motivator to prevent negligence, not a legal trap to cause hardship. Choosing a time when you are physically and mentally ready to fast—even if it means waiting a couple of weeks—is much better than forcing it during a busy family visit. God values the quality and sincerity of your worship over a rushed completion.