-,we shall go on a picnic

关键词在于【shall】.
表示“未来”的虚拟语气,所以必须用should.
例子：
Should the weather be bad,we shall/will have to move the meeting to another date.
“were”代表的是“过去”的虚拟语气,如果要用were的话,句子就得改成：
Were the weather fine,we could/would have gone on a picnic.