What are your thoughts on why the millennials won’t work on weekends? Why they resist so hard to do it? We will be delivering soon an app stack & to be sure that we will be delivering on time. I told to my DEV team ( All millennials ) that we have to work the upcoming weekends. Their reaction was unbelievable. They fought me more of this than any other issue we’d faced before.

Mayank, assuming we're talking about an ethical, "at-will" salaried employment relationship, we cannot expect that the number of hours worked is a direct measure of how loyal or appreciative one's employer will be.   

On a given team, the capacity to address a business need is going to be about individuals, skills, and results, and less about cultural or generational factors.  There are multiple dimensions of individuals' capabilities and needs at play in this situation.  We're all making choices based on many factors.

There's also the question of how a given job or work challenge maps to each individual's (Maslow's) hierarchy of needs and what he/she might be doing instead, if not being asked to work late.  If an employee loves the work, the choice might be easy.   If there are competing family or life objectives, the choice may be different.   Not surprisingly, the shift to work-at-home has also shifted the choices we're making about putting in extra hours.  

And how secure does that employee feel about  their job or compensation - this could also be a factor.   Again I'm assuming an ethical employer who does not encourage or exploit employees' insecurity about their jobs. 

Look at the situation from the employer's perspective.  Ultimately employees and contractors will be retained and rewarded based on their perceived value, and the difficulty of replacing them.   

Often this doesn't look "fair", but I would posit that many employees' contributions are not seen by their peers or, or their peers are focusing on the wrong metrics.

A manager/employer is going to assemble the best team possible given the situation.  Depending on the outcome, the employer might be motivated to consider other options like hiring/training additional personnel including outsourced assistance.  The employer doesn't have a choice.  Deliver what the customer expects, or a competitor will.

Agreed. Thanks for the thoughtful response Harry. This creates an interesting conflict in the workplace though as people collaborate on projects and have different expectations.

How do you balance this at work?

Expectations should almost certainly be set by employer (and employee) at the beginning of their employment at an organization -- during the interview as well as when an offer is proffered.

 Interview: "You are aware that this position can involve the periodic need for after hours and/or weekend work to meet dates for deliverables, etc.  Is this ok with you?"  

When the offer is being made:  "We want to be fully transparent and up front, this is a salary position and will require additional work after hours or on weekends from time to time.  Our offer is contingent upon your acceptance of that condition'.  

Then after the employee accepts the offer and before they begin work you may wish to reinforce that condition of the position  (you may even wish to have them sign an agreement in writing).

Interesting point Ronny. Per 's point above, do you think it unfair if someone does go beyond the 9-5pm and is consistently rewarded for doing so?

Well... to be honest, the initial question annoyed me. It's stating that a whole class of people behaves in a certain way, and is very arrogant. I've worked with millennials and older people. Some were great, some were not. 
I agree with Arthur's comment completely.
I also think that you should not expect people to be your slaves at work.