April 18 is just another spring day.  Find a therapist you can send clients to, so they can get over their brainwashing to an IRS "deadline" that doesn't exist.  Then when you finish their returns, get some OCD therapy yourself.  

We told clients that if they got everything to us by March 18, we expect to have their returns finished by April 18.  This week many more are arriving, with documents that they have had in their possession for two to four weeks.  Most of them will be refund or zero-balance returns.  For those who expect to owe, we will prepare extensions and tell them where to mail their check (or pay online).  Yes, we'll file extensions for everyone.  Doesn't take much time. 

If they don't like it, they won't come back next year.  That leaves time for reasonable people.  The ones we have been turning away for years because we didn't have time for new clients.  Life is so much easier when you don't live in a fantasy world of "the sky is falling on April 19."