Have just agreed a price on a second hand property, was going to start looking around for furniture but don't want to get stuck with a load of furniture having to store it if for some reason the seller pulls out.
Legally, at any time up to the exchange of signed contracts. In real life, anything can happen right up to the last moment when funds are handed over in exchange for vacant possession.
Wait until deal is done and dusted and you have vacant possession before buying anything. Its far easier (e.g.) to stay in rented accommodation and furnish the new property quickly after completion than to try and work everything around a perceived closing date that may or may not happen.