“There's only us, there's only this / Forget regret, or life is yours to miss / No other road, no other way / No day but today!”

Source: Rent

For Whom the Bell Tolls (1940)

Source: Postcards from Ed: Dispatches and Salvos from an American Iconoclast

Source: Life, the Truth, and Being Free (2010), p. 109
Context: How would your life be different if … You walked away from gossip and verbal defamation? Let today be the day … You speak only the good you know of other people and encourage others to do the same.

As quoted in Journal of France and Germany (1942–1944) by Gilbert Fowler White, in excerpt published in Living with Nature's Extremes: The Life of Gilbert Fowler White (2006) by Robert E. Hinshaw, p. 62. From the context http://books.google.com/books?id=_2qfZRp9SeEC&lpg=PP1&pg=PA62#v=onepage&q&f=false it seems that White did not specify whether he had heard Einstein himself say this or whether he was repeating a quote that had been passed along by someone else, so without a primary source the validity of this quote should be considered questionable.
Some have argued that elsewhere Einstein defined a "miracle" as a type of event he did not believe was possible—Einstein on Religion by Max Jammer (1999) quotes on p. 89 from a 1931 conversation Einstein had with David Reichinstein, where Reichinstein brought up philosopher Arthur Liebert's argument that the indeterminism of quantum mechanics might allow for the possibility of miracles, and Einstein replied that Liebert's argument dealt "with a domain in which lawful rationality [determinism] does not exist. A 'miracle,' however, is an exception from lawfulness; hence, there where lawfulness does not exist, also its exception, i.e., a miracle, cannot exist." ("Dort, wo eine Gesetzmässigkeit nicht vorhanden ist, kann auch ihre Ausnahme, d.h. ein Wunder, nicht existieren." D. Reichenstein, Die Religion der Gebildeten (1941), p. 21). However, it is clear from the context that Einstein was stating only that miracles cannot exist in a domain (quantum mechanics) where lawful rationality does not exist. He did not claim that miracles could never exist in any domain. Indeed, Einstein clearly believed, as seen in many quotations above, that the universe was comprehensible and rational, but he also described this characteristic of the universe as a "miracle". In another example, he is quoted as claiming belief in a God, "Who reveals Himself in the lawful harmony of the world."
As quoted in From Yale to Jail: The Life Story of a Moral Dissenter (1993) by David T. Dellinger, p. 418
Disputed
Variant: There are only two ways to live your life. One is as though nothing is a miracle. The other is as though everything is a miracle.
Variant: There are only two ways to live your life. One is as though nothing is a miracle. The other is as though everything is a miracle.

Reverence for Life (1969)

Source: Life, the Truth, and Being Free (2010), p. 119
Context: How would your life be different if… You stopped allowing other people to dilute or poison your day with their words or opinions? Let today be the day … You stand strong in the truth of your beauty and journey through your day without attachment to the validation of others.

Ibid., p. 50
The Book of Disquiet
Original: Um tédio que inclui a antecipação só de mais tédio; a pena, já, de amanhã ter pena de ter tido pena hoje.