Buenos Aires is in the Southern Hemisphere, so its seasons are the reverse of Europe and North America. The best times to visit are the mild shoulder seasons of autumn (March to May) and spring (September to November), when temperatures are pleasant, humidity is lower, and spring brings the city's jacaranda trees into purple bloom. Summer (December to February) is hot, humid and often steamy, and many locals leave the city for the coast, though it is lively for nightlife. Winter (June to August) is cool rather than cold, grey at times but rarely freezing, and the quietest and often cheapest season. Whenever you come, remember that Argentines live late — dinner after 9pm, nightlife past midnight.
Pickpocketing and bag-snatching are the main risk for visitors, especially in crowds, on public transport, around tourist spots like La Boca, San Telmo's Sunday fair and Florida street, and at cafe tables where phones are left out. Carry only what you need, keep bags zipped and in front of you, use a hidden money belt for spares, and stay alert to distraction tricks.
Political demonstrations and marches (manifestaciones) are common, often around Plaza de Mayo, the Congreso and the main avenues, and are usually peaceful but can disrupt traffic and occasionally turn tense. Avoid getting caught in the middle of a protest, follow local news, and give large gatherings a wide berth.
Buenos Aires is generally welcoming, but some areas warrant care: parts of La Boca outside the tourist streets, Retiro around the bus station and some southern barrios can be sketchy, especially after dark. Ask your accommodation which streets to avoid, take a taxi or app rather than walking alone late at night, and keep valuables out of sight.