1) I'm surprised you had to be wait to be told to go to open days, everyone I know has gone from their own initiative.
2) Open days are also sometimes held in the autumn so you may still have time to go before you send off your application.
3) Some universities hold applicant open days, open days that you can go to after you've applied.
4) Open days are fairly important but not essential so if you absolutely can't make it to one it's not the end of the world but try and go on another day, take a look around, ask for a map or information etc at the main reception. I'm not speaking for every university but it being booked full may not really matter. Two of my friends went to a 'full' open day but there was no issue with them being there. Some people change their minds or 'full' is just at the limit of ideal because they don't want to be overwhelmed.