Advanced Entrance Tickets

If you are positive you will be there on a certain date or time, IMO there is no such thing as too early to buy. You never know about certain sites that will fill up, reach a quota or that time will disappear.

as soon as tickets are available to purchase.

Well, I disagree. That's allowing an awful lot of time for a new, nasty variant to crop up.

La Sagrada Familia doesn't usually sell out more than a day or so in advance. Unless something has changed massively, I think buying a week ahead of time would be plenty early. I walked past LSF in the mid- to late-afternoon during May 2019. There was a long line of people--I assumed waiting to buy tickets--certainly proof that you're foolish not to buy one a bit in advance, but also an indication that there were probably still same-day tickets available at that point. If you are determined to have the first time slot of the morning, I guess a slightly earlier purchase might be prudent. In that case, I'd monitor ticket availability for dates a week or two ahead of mine, as well as for my date(s) to be sure nothing strange is happening (like a local festival bringing a lot of extra people into town). LSF is currently selling tickets through the end of May. Adult (non-senior) tickets are 26 euros each. That's more money than I'd want to risk unnecessarily.

Edited to add: There are other sights in Barcelona for which tickets need to be purchased in advance because ticket lines are so long: Parc Guell, Casa Mila/La Pedrera, Casa Batllo, Picasso Museum and Palau de la Musica Catalana. At the latter, the issue in the past has been the timing of the English-language tours.

[Revised to correct info on how far out tickets can be purchased. >>] The Alhambra has sold out a month or more ahead of time in the past, and it could certainly start doing so again by October. At the moment, tickets can be purchased for dates through December 31, and there is just one sold-out day during that period (this coming Saturday). The color-coding to indicate lower availability of tickets appears not to be working past May 2; every date is flagged yellow, which obviously is not reasonable. However, before May 2, there are quite a few days with low availability, which indicates you might not have a lot of choice about the time of your visit. The Alhambra is a large complex where one can spend nearly an entire day (though most people do not), so I would want a morning entry time. You can click on the date and see how many tickets remain for each time-block. There can be over a thousand tickets still remaining and the date showing "Last tickets".

The basic ticket for a full, daytime Alhambra visit is the "Alhambra General". Do not choose the less expensive "Gardens, Generalife and Alcazaba" ticket, because it omits the most fabulous sight, the Nasrid Palaces. Depending on your interests, one of the more expensive tickets might be right for you.

Acraven,

I just bought my Alhambra tickets today for May 23. When I went back and checked you can buy tickets through March 31, 2023. Almost every day is marked as "Last Tickets", meaning they are nearing sold out for that day, when in fact there are lots of tickets for all the dates I checked. Not sure why their website is showing that.

Honestly before Covid I think you would have been fine waiting to buy tickets for Sagrada Familia. However, people want to travel now and I wouldn't be surprised if times sell out. My advice is if you know day and time you want to go, buy them when they are available. So many people are traveling now that I fear the "old rules" of walking up and buying tickets are over.

The Alhambra I would buy the minute they are available, as the best days and times are usually booked quickly. My advice would be to go early in the day. I got there when they opened at 8:30 for a 10am ticket and had plenty of time walk around the area and take photos before the tour buses came.

Bill, you're right that the website allows you to buy tickets farther ahead than I thought, but despite the display of "Last Tickets" for literally every day from May 3 through December 31 (except for two days when the complex is closed), they are not running out of tickets. There are over 3000 tickets available right now for May 23. It appears the color-coding on the calendar intended to indicate ticket availability isn't working beyond May 2.

I'll update my earlier post to correct the misinformation about how far out tickets can currently be purchased.

We will be there in October this year

Where's "there"? You're vaguely naming two sites that are worlds apart and I presume you'll be visiting a few more, so I think I'd be more useful if you say exactly which sites and exactly when because responses will certainly vary.