*Solo* has an accent when it means "only" and can be mistaken with "alone":

Compare these two sentences:  

> - Voy a estar solo un día. = *I'm going to stay alone for a day.*
> - Voy a estar sólo un día. = *I'm going to stay just one day.*

When I learnt the rule, I was told "si puede sustituirse por *únicamente*, lleva tilde", but the actual rule is, "si puede sustituirse por *únicamente* y puede confundirse con *sin compañía*, lleva tilde". That said, I usually put the accent whenever it means "only".

You can find the [explanation by RAE in the DPD][1] (Diccionario panhispánico de dudas).

[1]: http://buscon.rae.es/dpdI/SrvltGUIBusDPD?origen=RAE&lema=tilde2#323