score:2
As pointed out by @Gagravarr in comments, take a look to posts on LoyaltyLobby in the category United Airlines Milage Plus.
These sales happened almost monthly (though many of those targeted) recently, but this being a volatile marketing instrument, you should not completely rely on another such sale coming up.
The sale you refer to apparantly was this one, apparently targeted and offering miles for sale at up to 75% bones w.r.t. the usual rate.
Since then (until today, 20 June 2016) there have been the following sales going on:
Do note, as pointed out on Loyaltylobby, that you can only
Purchase up to 150,000 miles per account per calendar year.