Database schema:

CREATE TABLE coins (
	id INTEGER NOT NULL -- example: ['2', '1'],
	name TEXT -- example: ['Bitcoin', 'Litecoin'],
	slug TEXT -- example: ['litecoin', 'bitcoin'],
	symbol TEXT -- example: ['LTC', 'BTC'],
	status TEXT -- example: ['active', 'untracked'],
	category TEXT -- example: ['token', 'coin'],
	description TEXT -- example: ['## **What Is Bitcoin (BTC)?**\\n\\nBitcoin is a dece', '## What Is Litecoin (LTC)?\\n\\nLitecoin (LTC) is a '],
	subreddit TEXT -- example: ['litecoin', 'bitcoin'],
	notice TEXT -- example: ['We have received reports that withdrawals for this', 'TIPS has undergone a [swap](https://bitcointalk.or'],
	tags TEXT -- example: ['mineable, pow, sha-256, store-of-value, state-chan', 'mineable, pow, scrypt, medium-of-exchange, binance'],
	tag_names TEXT -- example: ['Mineable, PoW, Scrypt, Medium of Exchange, Binance', 'Mineable, PoW, SHA-256, Store of Value, State chan'],
	website TEXT -- example: ['https://litecoin.org/', 'https://bitcoin.org/'],
	platform_id INTEGER -- example: ['1027', '1765'],
	date_added TEXT -- example: ['2013-05-03T00:00:00.000Z', '2013-04-28T00:00:00.000Z'],
	date_launched TEXT -- example: ['2018-05-01T00:00:00.000Z', '2018-01-16T00:00:00.000Z'],
	PRIMARY KEY (id)
)


CREATE TABLE historical (
	date DATE -- example: ['2013-04-28', '2013-04-29'],
	coin_id INTEGER -- example: ['2', '1'],
	cmc_rank INTEGER -- example: ['2', '1'],
	market_cap REAL -- example: ['1488566971.9558687', '74637021.56790735'],
	price REAL -- example: ['4.34840488433838', '134.210021972656'],
	open REAL -- example: ['4.366759777069092', '134.44400024414062'],
	high REAL -- example: ['4.573599815368652', '147.48800659179688'],
	low REAL -- example: ['134.0', '4.225639820098877'],
	close REAL -- example: ['4.383900165557861', '144.5399932861328'],
	time_high TEXT -- example: ['13:15:01', '13:15:03'],
	time_low TEXT -- example: ['19:25:02', '05:20:01'],
	volume_24h REAL -- example: ['46862728.0', '0.0'],
	percent_change_1h REAL -- example: ['0.639231', '0.799273'],
	percent_change_24h REAL -- example: ['7.63553', '0.920718'],
	percent_change_7d REAL -- example: ['-13.809', '-16.7201'],
	circulating_supply REAL -- example: ['17164230.0', '11091325.0'],
	total_supply REAL -- example: ['17164230.0', '11091325.0'],
	max_supply REAL -- example: ['84000000.0', '21000000.0'],
	num_market_pairs INTEGER -- example: ['5146', '7050']
)

Question: When is the best time to purchase Bitcoin?
Evidence: lowest price refers to low; best time refers to date with the low;

SQL:
SELECT T2.date FROM coins AS T1 INNER JOIN historical AS T2 ON T1.id = T2.coin_id WHERE T1.name = 'Bitcoin' ORDER BY T2.low LIMIT 1