Were. Use "were" if you are talking about something that isn't true. You were not there. If it WAS raining yesterday (we assume this is true), then why is the patio dry?

Either One

Both of them are correct and it's the speaker's choice which one to use. Native-speakers who are a bit older generally use 'were' (the subjunctive), but younger people often choose 'was'. NOTE: we only tend to use the subjunctive with the verb 'be', and we use past simple with other verbs. If only I HAD more free time. If only you DIDN'T live in Australia! If only he WERE older. If only he WAS older.