SteamDB isn't an official Valve site but it tracks player numbers and other information based on data which Steam makes publicly available. According to their tracking Dota 2 was the first Steam game to tip over the 1 million mark and that happened on Valentine's Day.
Looking at the peak concurrent user graph for the last 12 months, Dota's been nudging towards the million mark each weekend since the end of last year (965,572 on November 22, 2014, for example) but what I suspect might have pushed it over this particular weekend is the Year Beast event.
The Year Beast is a game mode which shows up for a ten-minute slot several times a day and you only get an hour's warning in-game that the event is about to drop. As with every new or temporary game mode there's enthusiasm and arguing to be had but I tried it out just now and it feels like the servers are getting well and truly hammered whenever the beast shows up.
The first few minutes of my Year Beast game were peppered with auto-disconnect warnings, lag and freezes. It coincides with a 1,070,904 spike being reported by SteamDB about that time. It seemed to settle down a bit after that but friends have reported similar signs of the infrastructure straining to keep up when they've played.
Looking at the graphs, as you would expect there are spikes in usage at weekends but it tends to fall back down on Mondays. Looking at today's graph, the peak concurrent usership is even bigger, hitting 1,149,032 at 2pm (another Year Beast popped up then) and with the potential for another event taking place after UK and US working hours.
For a spot of perspective in terms of the MOBA scene, though, Riot Games report that League of Legends gets over 7.5 million people playing at the same time during peak hours
For more Year Beast info I might try to marshal my thoughts, not least so I can tell Francis The Year Beast exactly what I think of his sarcastic use of the phrase "Well Played!"