Postpone.
Schedule the celebration before your friends do, to a date when you'll have money, preferably payday to keep the celebration's purpose, meaning and excitement.
Let's go celebrate on my first payday, it will be awesome!
As a friend, I would respect that and I think your friends will respect that too, a nice additional gesture to reinforce the message and convey you appreciate their support would be to bring something for them to eat too, that could mean making the nicest sandwiches or something your pantry and your knowledge of your friends' allows. Include notes with a personal touch and hand them early.
"Thank you for all the support, working together is gonna be great!"
Or something more personal, you know them better. If your friends are the type of people who are not uncomfortable eating in adequate public spaces you could make a picnic basket, and that would still be cool.
Although considering the type of friendship you described you have, they might still want to take you out tomorrow but who knows if they'd tell you, don't bring the basket if that's the case as to avoid any undesirable ripple effect, by now you have already shown you don't want to be a burden and you are willing to reciprocate to their nice gestures.
A respectable way to handle lunch time tomorrow if they take you out anyways, would be to make them company to maintain the excitementmaintain the excitement, eat something inexpensive and if by any chance you were not satisfied, well, you still have your sandwich waiting for you.
Best of luck with your sensitive situation and congratulations on the new job!
