How would I automatically change the closed date of an opportunity to the date on the event of the stage being changed to closed won or closed lost?

When we enter a new Lead into Salesforce we have a lookup field to add the Campaign rather than having to add the Lead then go to the add Camaign separately. Once the Lead has been saved the Campaign lookup is no longer visible in the Lead page layout.  Sometimes the Campaign is not known at the time of adding the Lead, however we do need to know the Campaign prior to the Lead being converted.  This can be done by adding the Campaign as normal, however our sales people do not always remember to do this. 

I looked at maybe making it a required field on the Converting page however you cannot change the converting page layout.  I have been advised by Salesforce Support to create a Validation Rule to make the Campaign a required field upon conversion, however they do not support validation rules.  I am not sure how to do this.  Can anyone help with how to make the Campaign lookup field, that is visible in the entering of a lead stage, a required field on converting?

There may also be another way to do this.  I am open to suggestions, I just need to make sure that there is an associated Campaign prior to the lead being converted without having to rely on the user to enter it.

Thank you.

I have not used formulas before and I am trying to create a Workflow to update a date field in Accounts with the Lead Converted Date on creation of the Account.

I want to create a View in Accounts to use for Mail Merges.  The Mail Merge needs to be sent to all new Accounts from Converted Leads and also existing Accounts that need to have the document resent.  I propose that it is a date field so that the view can show all Accounts with this field showing Today's date.  This will then show all the Accounts with the Leads that were converted today and then anyone can also update this field on any other Accounts with Today's date if they need to information resent.

I currently have a checkbox field so that the View is run on all Accounts with the checkbox checked but this is not working for me because I then have to manually uncheck the checkbox for every Account each day so they do not appear in the View for the next day.

I have created the workflow but I am not sure what the formula should be.

Thank You.

How would I automatically change the closed date of an opportunity to the date on the event of the stage being changed to closed won or closed lost?

I have not used formulas before and I am trying to create a Workflow to update a date field in Accounts with the Lead Converted Date on creation of the Account.

I want to create a View in Accounts to use for Mail Merges.  The Mail Merge needs to be sent to all new Accounts from Converted Leads and also existing Accounts that need to have the document resent.  I propose that it is a date field so that the view can show all Accounts with this field showing Today's date.  This will then show all the Accounts with the Leads that were converted today and then anyone can also update this field on any other Accounts with Today's date if they need to information resent.

I currently have a checkbox field so that the View is run on all Accounts with the checkbox checked but this is not working for me because I then have to manually uncheck the checkbox for every Account each day so they do not appear in the View for the next day.

I have created the workflow but I am not sure what the formula should be.

Thank You.