The could be called particular/specific events, but facts/occurrences may be best given the use of "ACONTECIMIENTOS".

For example, apart from betting on who will win the Manchester United vs Manchester City game, you can make side/proposition bets on who will score first, the scoreline, etc.

Proposition bets are wagers made on a very specific outcome of a match. Examples include guessing the number of goals each team scores in a handball match, betting whether a player will score in a football game, or wagering that a baseball player on one team will accumulate more hits than another player on the opposing team.
http://en.wikipedia.org/wiki/Sports_betting

These are great because if you are watching a game and you can see that the tide is turning in a particular direction you can bet on occurrences such as who is about to score or who is about to win a corner.
http://www.thepremier-league.co.uk/page/2

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Ladbrokes, alongside other betting companies, used the World Cup as a means to promote its real-time, or “in-running” betting offering, whereby punters can bet on particular events during a game rather than just on its eventual outcome.
http://www.ft.com/cms/s/0/f51d6268-d50c-11df-ad3a-00144feabd...

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Note added at 1 hr (2012-03-05 16:14:35 GMT)
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These terms are used in several gaming regulations I translated, including "fixed odds sports betting" and "fixed odds horse racing", e.g.
Sports betting:

En todo caso deberá garantizarse el derecho al cobro de los premios que pudieran haberse obtenido por los participantes por apuestas realizadas sobre HECHOS o CIRCUNSTANCIAS de los EVENTOS deportivos que se hubieran sustanciado con anterioridad a que se produjese la eventual suspensión o anulación.

And in “Fixed odds horseracing”:

Información, en su caso, sobre todas aquellas circunstancias que, a juicio del operador, puedan influir en el resultado del EVENTO o en los HECHOS o CIRCUNSTANCIAS sobre los que recaigan los pronósticos.
So taking into account that, throughout these regulations, these words appear (often together in the same sentences), they seem not to be synonomous. i.e. One means the actual event taking place (e.g. the horserace) and the “hechos” are cetain (particular) facts/occurences on which bets may be placed within the Event itself.