Nintendo is targeting the second half of 2020, because otherwise they wouldn't have made the announcement at this time. Switch has an abundance of upcoming first party games in the near future and if Zelda couldn't possibly be ready in 2020, they would have used a different team's game to have a big title for people to look forward to. Remember, the teams that released MK8D/ARMS, Splatoon 2 and Super Mario Odyssey in 2017 could all have their next game ready in 2020, but Nintendo picked the Zelda team for this announcement.
Considering that Pokémon was/is a holiday title in 2018 and 2019, it's highly probable that 2020 will be a year without Pokémon. Good chance that Zelda will be positioned as the big holiday title, so November 2020 is the most likely month. While a delay is possible, I think this time the Zelda team will deliver on time.