Because you have expressed an interest in learning about the verb "wish", you may want to know about the "wish subjunctive". In many uses, the verb "wish" expresses a desire for something that is not currently true. In those cases, many people use the subjunctive mood (particularly in formal English). Using the subjunctive mood, your sentence would be "I wish it were a nicer day today." It is my understanding that the subjunctive mood is used less often in BrE than it is in AmE.