Valencia-Celta is scheduled on 4/3/07.
On Saint Joseph's Day (19th March), here in Valencia we celebrate the fallas, a Valencian tradition in wich each neighbourhood of the city organize parties, dinners and the most important, the construction of a falla, a wooden monument which is eventually burnt.
So, IMHO, better to attend the Valencia-Racing Santander on 18/3/07. Thus you can combine football and the fallas festival.
Sold-outs at Mestalla are traditionally only the games against Barcelona and Real Madrid. It is possible always to buy tickets for all other matches. The only problem is that if you buy last-minute tickets, you may not get good places.
Of course, I can buy the tickets for you. Just let me know your preferences.