This year the first day of Fallas is sunday 16, but most of then are already set up by day 15 by night, and most important and bigger takes several days to mount, so on 15 saturday you'll see 'much' of many fallas.
Anyway things ou can do on saturday and sunday:
Go to the Mascleta at the Plaza del ayuntamiento, stay there at least one hour before. If you go on saturday i'll bet you'll go again on sunday,Mascletas are quite impressive fireworks.
At 12pm of saturday night, see the fireworks at the Alameda (at the old bed of the river).
If it's a warm day you can go to the beach, nearest and quite good beach is Malvarrossa and Las Arenas (are the same beach indeed), take a bus or a Tram to get there. gandia it's goood but you'll loose much of your time going there, Malvarrossa is in the same city of Valencia, but quite far if you are walking. It's also a good place to eat a Paella.
In the night, just walk in the center of Valencia, see the aleready set up fallas and join to any party you may find. Fallas are an open party, and being saturday i think that many fallas will put some music on the streets. Also you may go to a Cafe Paris and drink some Agua de Valencia.
Enjoy with Las Fallas!
Check also
www.fallas.com