JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
Yes, you can sign up for a monthly subscription and just downgrade back to "Free" plan if the event or registration is over.
In your case, if you just want it for 1 month, then just downgrade before the next billing date will happen. If you want it for 2 months, then just downgrade before the 3rd monthly charge, and so on...
Here's a guide on how to upgrade to a paid plan: https://www.jotform.com/help/331-How-to-Upgrade-My-Account
Let us know if you need more information on this.