Correction: National Gallery in TripAdvisor corrected me: the museum offers Open-House ONLY on SELECTED Public Holidays (which are announced in their website), not all public holidays.
Christmas free treat (as a foreigner): https://www.tripadvisor.com.sg/ShowUserReviews-g294265-d8077179-r452873577-National_Gallery_Singapore-Singapore.html#REVIEWS
I highly recommend everyone to visit the extensive Southeast Asian art pieces in the National Gallery Singapore. Standard ticket for Non-Singaporeans is 20 SGD, which is not expensive given the exhibits offered at each of the ~7 galleries. The museum is FREE FOR ALL during selected public holidays, which are announced in their website. So I was finally able to visit during the Christmas holidays, and it was an enriching experience and unique treat for myself. I was able to see paintings by Fernando Amorsolo, Felix Hidalgo and a Juan Luna! I would like to go back after I get to know more about other pioneering artists in S.E. Asia like Raden Saleh, Liu Kang... and acclaimed modern and contemporary artists like Norberto Roldan... The architecture of the former Supreme Court and City Hall is also something to behold, and I was also able to see Singapore's historical documents at the former Chief Justice's Chamber.
Here are some of my favourites during my visit (I wasnt able to finish from Level 2 to Basement galleries):
And here are some useful links I have found and is worth exploring!
The Filipino Collection in the National Gallery Singapore by Manila Art Scene
National Heritage Board, Singapore at Google Arts & Culture
National Gallery Singapore Website