Below is a round-up of all the regular free museum days (or hours) across New York:

ALWAYS FREE (or pay what you wish!) 

FREE DAYS/HOURS 

• Brooklyn Children's Museum (second weekend of the month, before 11 am) 
• Bronx Museum (first Friday, 6-10 pm) 
• Brooklyn Museum (first Saturday of the month, 5-11 pm)
• Museum of Arts and Design  (pay what you wish from 6-9 pm on Thursdays) 
• New Museum (free on Thursdays from 7-9 pm) 
• Museum of Modern Art (UNIQLO free Friday Nights - free on Fridays from 6:30 - 9 pm, children under 16 always get in free) 
• Whitney Museum of American Art (free on Fridays from 7-9:30 pm) 
• New York Botanical Garden (Free on Wednesdays and on Saturdays from 9-10 am) 
• Frick Collection (free on Wednesdays from 2-6 pm) 
• Museum of the Moving Image (free on Fridays from 4-8 pm) 
• Rubin Museum of Art (free on Fridays from 6-10 pm) 
• Guggenheim (pay as you wish on Saturdays from 5-8 pm, children under 12 are always free) 

• METROPOLITAN MUSEUM OF ART: The best bang-for-your-buck membership option at the Met is the “Member with Evening Hours” option. For $210 a year, you get two members cards plus two guests every visit (plus children under 18). And as you would guess from its title, you also get access to the museum outside of just morning hours.
• THE GUGGENHEIM: Offers a family membership for just $160 per year (includes up to 5 children). With the Family Membership, you get free admission to family-specific programs and exclusive event invitations.
• MOMA: Family memberships at the Museum of Modern Art are $200 per year, and the program gets you unlimited free admission for two cardholders plus all children under the age of 18. You also get access to kid-friendly events like Little Member Mornings.
• WHITNEY MUSEUM: The Whitney doesn’t offer a family membership, but Dual Memberships are $140 (go for the auto-renew option to save even more, at $126/year)
• NEW MUSEUM: Dual/Family Memberships at the New Museum are $110 per year, and get you free unlimited admission (plus half-price tickets for two guests per visit).
• BROOKLYN MUSEUM:  A Dual Membership (at $125/year) is the way to go here, with no option for a full family membership.
• MUSEUM OF ARTS AND DESIGN: If you’re looking for yearlong admission, plus discounts to the MAD’s family events, purchase a family membership for $125.